oral.h

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/**********************************************************************
 * LeechCraft - modular cross-platform feature rich internet client.
 * Copyright (C) 2006-2014  Georg Rudoy
 *
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
 **********************************************************************/

#pragma once

#include <stdexcept>
#include <type_traits>
#include <memory>
#include <optional>
#include <boost/preprocessor/stringize.hpp>
#include <boost/preprocessor/tuple.hpp>
#include <QStringList>
#include <QDateTime>
#include <QSqlQuery>
#include <QSqlRecord>
#include <QVariant>
#include <QtDebug>
#include <util/sll/ctstringutils.h>
#include <util/sll/prelude.h>
#include <util/sll/typelist.h>
#include <util/sll/typegetter.h>
#include <util/sll/void.h>
#include <util/db/dblock.h>
#include <util/db/util.h>
#include "oraltypes.h"
#include "sqliteimpl.h"

#ifndef ORAL_ADAPT_STRUCT

#define ORAL_STRING_FIELD_IMPL(index, sname, field)                                                                         \
    template<> \
    constexpr auto MemberNameByIdx<sname, index> = CtString { BOOST_PP_STRINGIZE(field) }; \
    template<> \
    constexpr auto MemberNameByPtr<&sname::field> = CtString { BOOST_PP_STRINGIZE(field) };

#define ORAL_STRING_FIELD(_, index, args) \
    ORAL_STRING_FIELD_IMPL(index, BOOST_PP_TUPLE_ELEM(0, args), BOOST_PP_TUPLE_ELEM(index, BOOST_PP_TUPLE_ELEM(1, args)))

#define ORAL_ADAPT_STRUCT(sname, ...)                                                                               \
namespace LC::Util::oral                                                                                            \
{                                                                                                                   \
    template<>                                                                                                      \
    constexpr auto BeenAdapted<sname> = true;                                                                       \
                                                                                                                    \
    template<>                                                                                                      \
    constexpr auto SeqSize<sname> = BOOST_PP_TUPLE_SIZE((__VA_ARGS__));                                             \
                                                                                                                    \
    static_assert (SeqSize<sname> == detail::FullSize<sname>);                                                      \
                                                                                                                    \
    BOOST_PP_REPEAT(BOOST_PP_TUPLE_SIZE((__VA_ARGS__)), ORAL_STRING_FIELD, (sname, (__VA_ARGS__)))                  \
}
#endif

namespace LC::Util::oral
{
    using QSqlQuery_ptr = std::shared_ptr<QSqlQuery>;

    class QueryException : public std::runtime_error
    {
    public:
        using std::runtime_error::runtime_error;

        ~QueryException () noexcept = default;
    };

    template<typename>
    constexpr bool BeenAdapted = false;

    template<typename>
    constexpr size_t SeqSize = 0;

    namespace detail
    {
        struct UnspecializedSentinel {};
    }

    template<typename, int>
    constexpr auto MemberNameByIdx = detail::UnspecializedSentinel {};

    template<auto>
    constexpr auto MemberNameByPtr = detail::UnspecializedSentinel {};

    namespace detail
    {
        template<size_t Idx, typename Seq>
        constexpr decltype (auto) Get (const Seq& seq)
        {
            const auto& [...fields] = seq;
            return (fields... [Idx]);
        }

        template<size_t Idx, typename Seq>
        constexpr decltype (auto) Get (Seq& seq)
        {
            auto& [...fields] = seq;
            return (fields... [Idx]);
        }

        template<typename Seq>
        constexpr auto GetFullSize (const Seq& seq)
        {
            const auto& [...fields] = seq;
            return std::integral_constant<size_t, sizeof... (fields)> {};
        }

        template<typename Seq>
        constexpr auto FullSize = decltype (GetFullSize (std::declval<Seq> ()))::value;

        template<typename T, CtString str>
        consteval auto MorphFieldName ()
        {
            if constexpr (requires { T::template FieldNameMorpher<str> (); })
                return T::template FieldNameMorpher<str> ();
            else if constexpr (str.EndsWith ('_'))
                return str.template Chop<1> ();
            else
                return str;
        }

        template<typename Seq, int Idx>
        consteval auto GetFieldName ()
        {
            constexpr auto str = MemberNameByIdx<Seq, Idx>;
            return MorphFieldName<Seq, str> ();
        }

        template<typename S>
        constexpr auto SeqIndices = std::make_index_sequence<SeqSize<S>> {};

        template<typename S>
        constexpr auto FieldNames = []<size_t... Ix> (std::index_sequence<Ix...>) constexpr
            {
                return std::tuple { GetFieldName<S, Ix> ()... };
            } (SeqIndices<S>);

        template<typename S>
        constexpr auto BoundFieldNames = []<size_t... Ix> (std::index_sequence<Ix...>) constexpr
            {
                return std::tuple { (":" + GetFieldName<S, Ix> ())... };
            } (SeqIndices<S>);

        template<typename S>
        constexpr auto QualifiedFieldNames = []<size_t... Ix> (std::index_sequence<Ix...>) constexpr
            {
                return std::tuple { (S::ClassName + "." + GetFieldName<S, Ix> ())... };
            } (SeqIndices<S>);

        template<auto Ptr>
        constexpr auto FieldNameByPtr = MorphFieldName<MemberPtrStruct_t<Ptr>, MemberNameByPtr<Ptr>> ();

        template<auto Ptr>
        constexpr auto QualifiedFieldNameByPtr = MemberPtrStruct_t<Ptr>::ClassName + "." + FieldNameByPtr<Ptr>;

        template<typename T>
        concept TypeNameCustomizedMember = requires { typename T::TypeName; };

        template<typename T>
        concept BaseTypeCustomized = requires { typename T::BaseType; };
    }

    template<typename ImplFactory, typename T>
    struct Type2Name
    {
        constexpr auto operator() () const noexcept
        {
            if constexpr (detail::TypeNameCustomizedMember<T>)
                return T::TypeName;
            else if constexpr (detail::BaseTypeCustomized<T>)
                return Type2Name<ImplFactory, typename T::BaseType> {} ();
            else if constexpr (HasType<T> (Typelist<int, qlonglong, qulonglong, bool> {}) || std::is_enum_v<T>)
                return "INTEGER"_ct;
            else if constexpr (std::is_same_v<T, double>)
                return "REAL"_ct;
            else if constexpr (std::is_same_v<T, QString> || std::is_same_v<T, QDateTime> || std::is_same_v<T, QUrl>)
                return "TEXT"_ct;
            else if constexpr (std::is_same_v<T, QByteArray>)
                return ImplFactory::TypeLits::Binary;
            else
                static_assert (std::is_same_v<T, struct Dummy>, "Unsupported type");
        }
    };

    template<typename ImplFactory, typename T>
    struct Type2Name<ImplFactory, Unique<T>>
    {
        constexpr auto operator() () const noexcept { return Type2Name<ImplFactory, T> {} () + " UNIQUE"; }
    };

    template<typename ImplFactory, typename T>
    struct Type2Name<ImplFactory, NotNull<T>>
    {
        constexpr auto operator() () const noexcept { return Type2Name<ImplFactory, T> {} () + " NOT NULL"; }
    };

    template<typename ImplFactory, typename T, typename... Tags>
    struct Type2Name<ImplFactory, PKey<T, Tags...>>
    {
        constexpr auto operator() () const noexcept { return Type2Name<ImplFactory, T> {} () + " PRIMARY KEY"; }
    };

    template<typename ImplFactory, typename... Tags>
    struct Type2Name<ImplFactory, PKey<int, Tags...>>
    {
        constexpr auto operator() () const noexcept { return ImplFactory::TypeLits::IntAutoincrement; }
    };

    template<typename ImplFactory, typename T>
    struct Type2Name<ImplFactory, std::optional<T>>
    {
        constexpr auto operator() () const noexcept { return Type2Name<ImplFactory, T> {} (); }
    };

    template<typename ImplFactory, auto Ptr>
    struct Type2Name<ImplFactory, References<Ptr>>
    {
        constexpr auto operator() () const noexcept
        {
            constexpr auto className = MemberPtrStruct_t<Ptr>::ClassName;
            return Type2Name<ImplFactory, ReferencesValue_t<Ptr>> () () +
                    " REFERENCES " + className + " (" + detail::FieldNameByPtr<Ptr> + ") ON DELETE CASCADE";
        }
    };

    template<typename T>
        requires (!std::same_as<T, QVariant>)
    struct ConvertT;

    template<typename T>
    constexpr auto Convert = ConvertT<T> {};

    template<typename T>
        requires (!std::same_as<T, QVariant>)
    struct ConvertT
    {
        static QVariant operator() (const T& t) noexcept
        {
            if constexpr (detail::TypeNameCustomizedMember<T>)
                return t.ToVariant ();
            else if constexpr (detail::BaseTypeCustomized<T>)
                return Convert<typename T::BaseType> (t.ToBaseType ());
            else if constexpr (std::is_same_v<T, QDateTime>)
                return t.toString (Qt::ISODateWithMs);
            else if constexpr (std::is_enum_v<T>)
                return static_cast<qint64> (t);
            else if constexpr (IsIndirect<T> {})
                return Convert<typename T::value_type> (t);
            else
                return t;
        }

        static T operator() (const QVariant& var) noexcept
        {
            if constexpr (detail::TypeNameCustomizedMember<T>)
                return T::FromVariant (var);
            else if constexpr (detail::BaseTypeCustomized<T>)
                return T::FromBaseType (Convert<typename T::BaseType> (var));
            else if constexpr (std::is_same_v<T, QDateTime>)
                return QDateTime::fromString (var.toString (), Qt::ISODateWithMs);
            else if constexpr (std::is_enum_v<T>)
                return static_cast<T> (var.value<qint64> ());
            else if constexpr (IsIndirect<T> {})
                return Convert<typename T::value_type> (var);
            else
                return var.value<T> ();
        }
    };

    template<typename T>
    struct ConvertT<std::optional<T>>
    {
        static QVariant operator() (const std::optional<T>& t) noexcept
        {
            return t ? Convert<T> (*t) : QVariant {};
        }

        static std::optional<T> operator() (const QVariant& var) noexcept
        {
            return var.isNull () ? std::nullopt : std::optional { Convert<T> (var) };
        }
    };

    namespace detail
    {
        template<typename Seq, int Idx>
        using ValueAtC_t = std::decay_t<decltype (Get<Idx> (std::declval<Seq> ()))>;

        template<typename T>
        constexpr bool IsPKey = false;

        template<typename U, typename... Tags>
        constexpr bool IsPKey<PKey<U, Tags...>> = true;

        template<typename T>
        QVariant ToVariantF (const T& t) noexcept
        {
            return Convert<T> (t);
        }

        template<size_t Ix, typename Seq>
        void BindAtIndex (const Seq& seq, QSqlQuery& query, bool bindPrimaryKey)
        {
            if (bindPrimaryKey || !IsPKey<ValueAtC_t<Seq, Ix>>)
                query.bindValue (ToString<std::get<Ix> (BoundFieldNames<Seq>)> (), ToVariantF (Get<Ix> (seq)));
        }

        template<typename Seq>
        auto DoInsert (const Seq& seq, QSqlQuery& insertQuery, bool bindPrimaryKey)
        {
            [&]<size_t... Ix> (std::index_sequence<Ix...>)
            {
                (BindAtIndex<Ix> (seq, insertQuery, bindPrimaryKey), ...);
            } (SeqIndices<Seq>);

            if (!insertQuery.exec ())
            {
                qCritical () << "insert query execution failed";
                DBLock::DumpError (insertQuery);
                throw QueryException ("insert query execution failed");
            }
        }

        template<typename Seq>
        consteval int PKeyIndexUnsafe ()
        {
            auto run = []<size_t... Idxes> (std::index_sequence<Idxes...>)
            {
                int result = -1;
                ((IsPKey<ValueAtC_t<Seq, Idxes>> ? (result = Idxes) : 0), ...);
                return result;
            };
            return run (SeqIndices<Seq>);
        }

        template<typename Seq>
        consteval int PKeyIndex ()
        {
            const auto idx = PKeyIndexUnsafe<Seq> ();
            static_assert (idx >= 0);
            return idx;
        }

        template<typename Seq>
        constexpr int PKeyIndex_v = PKeyIndex<Seq> ();

        template<typename Seq>
        concept HasPKey = PKeyIndexUnsafe<Seq> () >= 0;

        template<typename Seq>
        constexpr auto HasAutogenPKey () noexcept
        {
            if constexpr (HasPKey<Seq>)
                return !HasType<NoAutogen> (AsTypelist_t<ValueAtC_t<Seq, PKeyIndex_v<Seq>>> {});
            else
                return false;
        }

        template<typename Seq>
        constexpr auto ExtractReplaceFields (InsertAction::Replace::WholeType)
        {
            constexpr static auto allNames = FieldNames<Seq>;
            if constexpr (HasAutogenPKey<Seq> ())
                return []<size_t... Idxs> (std::index_sequence<Idxs...>)
                {
                    const auto [...namesPack] = allNames;
                    return std::tuple { namesPack... [Idxs >= PKeyIndex_v<Seq> ? Idxs + 1 : Idxs]... };
                } (std::make_index_sequence<SeqSize<Seq> - 1> ());
            else
                return allNames;
        }

        template<typename Seq, auto... Ptrs>
        constexpr auto ExtractReplaceFields (InsertAction::Replace::FieldsType<Ptrs...>)
        {
            return std::tuple { detail::FieldNameByPtr<Ptrs>... };
        }

        template<typename Seq>
        class AdaptInsert
        {
            const QSqlDatabase DB_;
            constexpr static bool HasAutogen_ = HasAutogenPKey<Seq> ();
        public:
            AdaptInsert (const QSqlDatabase& db) noexcept
            : DB_ { db }
            {
            }

            template<typename Action = InsertAction::DefaultTag>
            auto operator() (Seq& t, Action action = {}) const
            {
                return Run<SQLite::ImplFactory> (t, action);
            }

            template<typename ImplFactory>
            auto operator() (ImplFactory, Seq& t, auto action) const
            {
                return Run<ImplFactory> (t, action);
            }

            template<typename Action = InsertAction::DefaultTag>
            auto operator() (const Seq& t, Action action = {}) const
            {
                return Run<SQLite::ImplFactory> (t, action);
            }

            template<typename ImplFactory>
            auto operator() (ImplFactory, const Seq& t, auto action) const
            {
                return Run<ImplFactory> (t, action);
            }
        private:
            template<typename ImplFactory, typename Action>
            constexpr static auto MakeInsertSuffix (Action action)
            {
                if constexpr (HasAutogen_)
                {
                    constexpr auto pkIdx = PKeyIndex_v<Seq>;
                    if constexpr (std::is_same_v<Action, InsertAction::DefaultTag> || std::is_same_v<Action, InsertAction::IgnoreTag>)
                        return ImplFactory::GetInsertSuffix (action, GetFieldName<Seq, pkIdx> ());
                    else
                        return ImplFactory::GetInsertSuffix (InsertAction::Replace {},
                                ExtractReplaceFields<Seq> (action),
                                GetFieldName<Seq, pkIdx> ());
                }
                else
                {
                    if constexpr (std::is_same_v<Action, InsertAction::DefaultTag> || std::is_same_v<Action, InsertAction::IgnoreTag>)
                        return ImplFactory::GetInsertSuffix (action);
                    else
                        return ImplFactory::GetInsertSuffix (InsertAction::Replace {},
                                ExtractReplaceFields<Seq> (action));
                }
            }

            template<typename ImplFactory>
            constexpr static auto MakeQueryForAction (auto action)
            {
                return ImplFactory::GetInsertPrefix (action) +
                        " INTO " + Seq::ClassName +
                        " (" + JoinTup (FieldNames<Seq>, ", ") + ") " +
                        "VALUES (" + JoinTup (BoundFieldNames<Seq>, ", ") + ") " +
                        MakeInsertSuffix<ImplFactory> (action);
            }

            template<typename ImplFactory, typename T, typename Action>
            auto Run (T& t, Action action) const
            {
                QSqlQuery query { DB_ };
                constexpr auto queryText = MakeQueryForAction<ImplFactory> (action);
                query.prepare (ToString<queryText> ());

                DoInsert (t, query, !HasAutogen_);

                if constexpr (HasAutogen_)
                {
                    constexpr auto pkIndex = PKeyIndex_v<Seq>;
                    constexpr auto mightBeMissing = std::same_as<Action, InsertAction::IgnoreTag>;
                    using FieldType_t = ValueAtC_t<Seq, pkIndex>;

                    if (!query.next ())
                    {
                        if constexpr (mightBeMissing)
                            return std::optional<FieldType_t> {};
                        throw QueryException { "unable to fetch last inserted rowid" };
                    }

                    const auto& lastId = Convert<FieldType_t> (query.value (0));

                    if constexpr (!std::is_const_v<T>)
                        Get<pkIndex> (t) = lastId;

                    if constexpr (mightBeMissing)
                        return std::optional { lastId };
                    else
                        return lastId;
                }
            }
        };

        template<typename Seq>
        struct AdaptDelete
        {
            QSqlQuery DeleteQuery_;
        public:
            AdaptDelete (const QSqlDatabase& db) noexcept
            : DeleteQuery_ { db }
            {
                if constexpr (HasPKey<Seq>)
                {
                    constexpr auto index = PKeyIndex_v<Seq>;
                    constexpr auto del = "DELETE FROM " + Seq::ClassName +
                            " WHERE " + std::get<index> (FieldNames<Seq>) + " = ?";
                    DeleteQuery_.prepare (ToString<del> ());
                }
            }

            void operator() (const Seq& seq) requires HasPKey<Seq>
            {
                constexpr auto index = PKeyIndex_v<Seq>;
                DeleteQuery_.bindValue (0, ToVariantF (Get<index> (seq)));
                if (!DeleteQuery_.exec ())
                    throw QueryException ("delete query execution failed");
            }
        };

        template<typename T, size_t... Indices>
        T InitializeFromQuery (const QSqlQuery& q, std::index_sequence<Indices...>, int startIdx) noexcept
        {
            if constexpr (requires { T { Convert<ValueAtC_t<T, Indices>> (QVariant {})... }; })
                return T { Convert<ValueAtC_t<T, Indices>> (q.value (startIdx + Indices))... };
            else
            {
                T t;
                ((Get<Indices> (t) = Convert<ValueAtC_t<T, Indices>> (q.value (startIdx + Indices))), ...);
                return t;
            }
        }

        enum class ExprType
        {
            ConstTrue,

            LeafStaticPlaceholder,
            LeafData,

            Greater,
            Less,
            Equal,
            Geq,
            Leq,
            Neq,

            Like,
            ILike,

            And,
            Or
        };

        template<ExprType Type>
        constexpr auto TypeToSql () noexcept
        {
            if constexpr (Type == ExprType::Greater)
                return ">"_ct;
            else if constexpr (Type == ExprType::Less)
                return "<"_ct;
            else if constexpr (Type == ExprType::Equal)
                return "="_ct;
            else if constexpr (Type == ExprType::Geq)
                return ">="_ct;
            else if constexpr (Type == ExprType::Leq)
                return "<="_ct;
            else if constexpr (Type == ExprType::Neq)
                return "!="_ct;
            else if constexpr (Type == ExprType::Like || Type == ExprType::ILike)
                return "LIKE"_ct;
            else if constexpr (Type == ExprType::And)
                return "AND"_ct;
            else if constexpr (Type == ExprType::Or)
                return "OR"_ct;
            else
                static_assert (std::is_same_v<struct D1, ExprType>, "Invalid expression type");
        }

        constexpr bool IsRelational (ExprType type) noexcept
        {
            return type == ExprType::Greater ||
                    type == ExprType::Less ||
                    type == ExprType::Equal ||
                    type == ExprType::Geq ||
                    type == ExprType::Leq ||
                    type == ExprType::Neq ||
                    type == ExprType::Like ||
                    type == ExprType::ILike;
        }

        template<ExprType Type>
        constexpr auto WrapSubexpr (auto subexpr)
        {
            if constexpr (Type == ExprType::ILike)
                return "lower(" + subexpr + ")";
            else
                return subexpr;
        }

        template<typename T>
        struct WrapDirect
        {
            using value_type = T;
        };

        template<typename T>
        using UnwrapIndirect_t = typename std::conditional_t<IsIndirect<T> {},
                T,
                WrapDirect<T>>::value_type;

        template<ExprType Type, typename Seq, typename L, typename R>
        constexpr bool Typecheck ()
        {
            if constexpr (IsRelational (Type))
            {
                using LReal = UnwrapIndirect_t<typename L::template ValueType_t<Seq>>;
                using RReal = UnwrapIndirect_t<typename R::template ValueType_t<Seq>>;
                return requires (LReal l, RReal r) { l == r; };
            }
            else
                return true;
        }

        template<ExprType Type, typename L = void, typename R = void>
        class ExprTree;

        template<typename T>
        constexpr bool IsExprTree = false;

        template<ExprType Type, typename L, typename R>
        constexpr bool IsExprTree<ExprTree<Type, L, R>> = true;

        template<typename L, typename R>
        class AssignList
        {
            L Left_;
            R Right_;
        public:
            constexpr AssignList (const L& l, const R& r) noexcept
            : Left_ { l }
            , Right_ { r }
            {
            }

            template<typename Seq, CtString S>
            constexpr static auto ToSql () noexcept
            {
                if constexpr (IsExprTree<L>)
                    return L::GetFieldName () + " = " + R::template ToSql<Seq, S + "r"> ();
                else
                    return L::template ToSql<Seq, S + "l"> () + ", " + R::template ToSql<Seq, S + "r"> ();
            }

            template<typename Seq, CtString S>
            void BindValues (QSqlQuery& query) const noexcept
            {
                Left_.template BindValues<Seq, S + "l"> (query);
                Right_.template BindValues<Seq, S + "r"> (query);
            }

            template<typename OL, typename OR>
            constexpr auto operator, (const AssignList<OL, OR>& tail) noexcept
            {
                return AssignList<AssignList<L, R>, AssignList<OL, OR>> { *this, tail };
            }
        };

        template<ExprType Type, typename L, typename R>
        class ExprTree
        {
            L Left_;
            R Right_;
        public:
            constexpr ExprTree (const L& l, const R& r) noexcept
            : Left_ (l)
            , Right_ (r)
            {
            }

            template<typename Seq, CtString S>
            constexpr static auto ToSql () noexcept
            {
                static_assert (Typecheck<Type, Seq, L, R> (),
                        "Incompatible types passed to a relational operator.");

                return WrapSubexpr<Type> (L::template ToSql<Seq, S + "l"> ()) +
                        " " + TypeToSql<Type> () + " " +
                        WrapSubexpr<Type> (R::template ToSql<Seq, S + "r"> ());
            }

            template<typename Seq, CtString S>
            void BindValues (QSqlQuery& query) const noexcept
            {
                Left_.template BindValues<Seq, S + "l"> (query);
                Right_.template BindValues<Seq, S + "r"> (query);
            }

            template<typename T>
            constexpr static auto AdditionalTables () noexcept
            {
                return std::tuple_cat (L::template AdditionalTables<T> (), R::template AdditionalTables<T> ());
            }

            template<typename T>
            constexpr static bool HasAdditionalTables () noexcept
            {
                return L::template HasAdditionalTables<T> () || R::template HasAdditionalTables<T> ();
            }
        };

        template<typename T>
        class ExprTree<ExprType::LeafData, T, void>
        {
            const T& Data_;
        public:
            template<typename>
            using ValueType_t = T;

            constexpr ExprTree (const T& t) noexcept
            : Data_ (t)
            {
            }

            template<typename, CtString S>
            constexpr static auto ToSql () noexcept
            {
                return ":bound_" + S;
            }

            template<typename Seq, CtString S>
            void BindValues (QSqlQuery& query) const noexcept
            {
                constexpr auto varName = ToSql<Seq, S> ();
                query.bindValue (ToString<varName> (), ToVariantF (Data_));
            }

            template<typename>
            constexpr static auto AdditionalTables () noexcept
            {
                return std::tuple {};
            }

            template<typename>
            constexpr static bool HasAdditionalTables () noexcept
            {
                return false;
            }
        };

        template<typename... Ts>
        class ExprTree<ExprType::LeafData, std::tuple<Ts...>, void>
        {
            const std::tuple<Ts...>& Data_;

            template<size_t Idx, CtString S>
            constexpr static auto BoundVarName = ":bound_" + S + IntegralToString<Idx> ();
        public:
            template<typename>
            using ValueType_t = std::tuple<Ts...>;

            constexpr ExprTree (const std::tuple<Ts...>& t) noexcept
            : Data_ (t)
            {
            }

            template<typename, CtString S>
            constexpr static auto ToSql () noexcept
            {
                return []<size_t... Indices> (std::index_sequence<Indices...>)
                {
                    return "(" + Join (", "_ct, BoundVarName<Indices, S>...) + ")";
                } (std::index_sequence_for<Ts...> {});
            }

            template<typename Seq, CtString S>
            void BindValues (QSqlQuery& query) const noexcept
            {
                [&]<size_t... Indices> (std::index_sequence<Indices...>)
                {
                    (query.bindValue (ToString<BoundVarName<Indices, S>> (), ToVariantF (std::get<Indices> (Data_))), ...);
                } (std::index_sequence_for<Ts...> {});
            }

            template<typename>
            constexpr static auto AdditionalTables () noexcept
            {
                return std::tuple {};
            }

            template<typename>
            constexpr static bool HasAdditionalTables () noexcept
            {
                return false;
            }
        };

        template<typename T>
        constexpr auto AsLeafData (const T& node) noexcept
        {
            if constexpr (IsExprTree<T>)
                return node;
            else
                return ExprTree<ExprType::LeafData, T> { node };
        }

        template<auto... Ptr>
        struct MemberPtrs {};

        template<auto Ptr>
        class ExprTree<ExprType::LeafStaticPlaceholder, MemberPtrs<Ptr>, void>
        {
            using ExpectedType_t = MemberPtrType_t<Ptr>;
        public:
            template<typename>
            using ValueType_t = ExpectedType_t;

            template<typename Seq, CtString S>
            constexpr static auto ToSql () noexcept
            {
                return MemberPtrStruct_t<Ptr>::ClassName + "." + GetFieldName ();
            }

            template<typename Seq, CtString S>
            void BindValues (QSqlQuery&) const noexcept
            {
            }

            constexpr static auto GetFieldName () noexcept
            {
                return detail::FieldNameByPtr<Ptr>;
            }

            template<typename T>
            constexpr static auto AdditionalTables () noexcept
            {
                using Seq = MemberPtrStruct_t<Ptr>;
                if constexpr (std::is_same_v<Seq, T>)
                    return std::tuple {};
                else
                    return std::tuple { Seq::ClassName };
            }

            template<typename T>
            constexpr static bool HasAdditionalTables () noexcept
            {
                return !std::is_same_v<MemberPtrStruct_t<Ptr>, T>;
            }

            constexpr auto operator= (const ExpectedType_t& r) const noexcept
            {
                return AssignList { *this, AsLeafData (r) };
            }
        };

        template<auto... Ptrs>
            requires (sizeof... (Ptrs) > 1)
        class ExprTree<ExprType::LeafStaticPlaceholder, MemberPtrs<Ptrs...>, void>
        {
            using ExpectedType_t = std::tuple<MemberPtrType_t<Ptrs>...>;
        public:
            template<typename>
            using ValueType_t = ExpectedType_t;

            template<typename Seq, CtString S>
            constexpr static auto ToSql () noexcept
            {
                return "(" + Join (", "_ct, (MemberPtrStruct_t<Ptrs>::ClassName + "." + FieldNameByPtr<Ptrs>)...) + ")";
            }

            template<typename Seq, CtString S>
            void BindValues (QSqlQuery&) const noexcept
            {
            }

            template<typename T>
            constexpr static auto AdditionalTables () noexcept
            {
                return std::tuple_cat ([]
                {
                    using Seq = MemberPtrStruct_t<Ptrs>;
                    if constexpr (std::is_same_v<Seq, T>)
                        return std::tuple {};
                    else
                        return std::tuple { Seq::ClassName };
                } ()...);
            }

            template<typename T>
            constexpr static bool HasAdditionalTables () noexcept
            {
                return !(std::is_same_v<MemberPtrStruct_t<Ptrs>, T> || ...);
            }
        };

        template<>
        class ExprTree<ExprType::ConstTrue, void, void>
        {
        public:
            template<typename, CtString>
            constexpr static auto ToSql () noexcept
            {
                return "1 = 1"_ct;
            }

            template<typename, CtString>
            void BindValues (QSqlQuery&) const noexcept
            {
            }

            template<typename>
            constexpr static bool HasAdditionalTables () noexcept
            {
                return false;
            }
        };

        constexpr auto ConstTrueTree_v = ExprTree<ExprType::ConstTrue> {};

        template<ExprType Type, typename L, typename R>
        auto MakeExprTree (const L& left, const R& right) noexcept
        {
            using EL = decltype (AsLeafData (left));
            using ER = decltype (AsLeafData (right));
            return ExprTree<Type, EL, ER> { AsLeafData (left), AsLeafData (right) };
        }

        template<typename L, typename R>
        concept EitherIsExprTree = IsExprTree<L> || IsExprTree<R>;

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator== (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Equal> (left, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator< (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Less> (left, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator<= (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Leq> (left, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator> (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Greater> (left, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator>= (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Geq> (left, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator!= (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Neq> (left, right);
        }

        template<ExprType Op>
        struct InfixBinary {};
    }

    namespace infix
    {
        constexpr detail::InfixBinary<detail::ExprType::Like> like {};
        constexpr detail::InfixBinary<detail::ExprType::ILike> ilike {};
    }

    namespace detail
    {
        template<typename L, ExprType Op>
        struct InfixBinaryProxy
        {
            const L& Left_;
        };

        template<typename L, ExprType Op>
        auto operator| (const L& left, InfixBinary<Op>) noexcept
        {
            return InfixBinaryProxy<L, Op> { left };
        }

        template<typename L, ExprType Op, typename R>
        auto operator| (const InfixBinaryProxy<L, Op>& left, const R& right) noexcept
        {
            return MakeExprTree<Op> (left.Left_, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator&& (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::And> (left, right);
        }

        template<typename L, typename R>
            requires EitherIsExprTree<L, R>
        auto operator|| (const L& left, const R& right) noexcept
        {
            return MakeExprTree<ExprType::Or> (left, right);
        }

        template<CtString BindPrefix, typename Seq, typename Tree>
        constexpr auto ExprTreeToSql () noexcept
        {
            return Tree::template ToSql<Seq, BindPrefix> ();
        }

        template<CtString BindPrefix, typename Seq, typename Tree>
        void BindExprTree (const Tree& tree, QSqlQuery& query)
        {
            tree.template BindValues<Seq, BindPrefix> (query);
        }

        enum class AggregateFunction
        {
            Count,
            Min,
            Max
        };

        template<AggregateFunction, auto Ptr>
        struct AggregateType {};

        template<typename FunRetType, CtString Fun, auto Ptr>
        struct CustomFunctionType {};

        struct CountAll {};

        template<typename... MemberDirectionList>
        struct OrderBy {};

        template<auto... Ptrs>
        struct GroupBy {};

        struct SelectWhole {};

        template<typename L, typename R>
        struct SelectorUnion {};

        template<typename T>
        struct SelectDistinct
        {
            explicit SelectDistinct (T) noexcept {}
        };

        template<typename T>
        constexpr bool IsSelector = false;

        template<>
        inline constexpr bool IsSelector<SelectWhole> = true;

        template<AggregateFunction Fun, auto Ptr>
        constexpr bool IsSelector<AggregateType<Fun, Ptr>> = true;

        template<typename FunRetType, CtString Fun, auto Ptr>
        constexpr bool IsSelector<CustomFunctionType<FunRetType, Fun, Ptr>> = true;

        template<auto... Ptrs>
        constexpr bool IsSelector<MemberPtrs<Ptrs...>> = true;

        template<typename L, typename R>
        constexpr bool IsSelector<SelectorUnion<L, R>> = true;

        template<typename T>
        constexpr bool IsSelector<SelectDistinct<T>> = true;

        template<typename L, typename R>
            requires IsSelector<L> && IsSelector<R>
        SelectorUnion<L, R> operator+ (L, R) noexcept
        {
            return {};
        }
    }

    namespace sph
    {
        template<auto Ptr>
        constexpr detail::ExprTree<detail::ExprType::LeafStaticPlaceholder, detail::MemberPtrs<Ptr>> f {};

        template<auto... Ptrs>
        constexpr detail::ExprTree<detail::ExprType::LeafStaticPlaceholder, detail::MemberPtrs<Ptrs...>> tuple {};

        template<auto... Ptrs>
        constexpr detail::MemberPtrs<Ptrs...> fields {};

        constexpr detail::SelectWhole all {};

        template<typename T>
        using distinct = detail::SelectDistinct<T>;

        template<auto... Ptrs>
        struct asc {};

        template<auto... Ptrs>
        struct desc {};

        // observe the irony: we count ⊤!
        template<auto Ptr = std::true_type {}>
        constexpr detail::AggregateType<detail::AggregateFunction::Count, Ptr> count {};

        template<auto Ptr>
        constexpr detail::AggregateType<detail::AggregateFunction::Min, Ptr> min {};

        template<auto Ptr>
        constexpr detail::AggregateType<detail::AggregateFunction::Max, Ptr> max {};

        template<typename FunRetType, CtString Fun, auto Ptr>
        constexpr detail::CustomFunctionType<FunRetType, Fun, Ptr> fun {};
    };

    template<typename... Orders>
    constexpr detail::OrderBy<Orders...> OrderBy {};

    template<auto... Ptrs>
    constexpr detail::GroupBy<Ptrs...> GroupBy {};

    struct Limit
    {
        uint64_t Count;
    };

    struct Offset
    {
        uint64_t Count;
    };

    namespace detail
    {
        template<auto Ptr>
        auto MemberFromVariant (const QVariant& var) noexcept
        {
            return Convert<UnwrapIndirect_t<MemberPtrType_t<Ptr>>> (var);
        }

        template<auto Ptr>
        auto MakeIndexedQueryHandler (const QSqlQuery& q, int startIdx = 0) noexcept
        {
            return MemberFromVariant<Ptr> (q.value (startIdx));
        }

        template<auto... Ptrs>
        auto MakeIndexedQueryHandler (MemberPtrs<Ptrs...>, const QSqlQuery& q, int startIdx) noexcept
        {
            if constexpr (sizeof... (Ptrs) == 1)
                return MakeIndexedQueryHandler<Ptrs...> (q, startIdx);
            else
                return [&]<size_t... Ix> (std::index_sequence<Ix...>)
                {
                    return std::tuple { MemberFromVariant<Ptrs> (q.value (startIdx + Ix))... };
                } (std::make_index_sequence<sizeof... (Ptrs)> {});
        }

        enum class SelectBehaviour { Some, One };

        struct OrderNone {};
        struct GroupNone {};
        struct LimitNone {};
        struct OffsetNone {};

        template<size_t RepIdx, size_t TupIdx, typename Tuple, typename NewType>
        constexpr decltype (auto) GetReplaceTupleElem (Tuple&& tuple, NewType&& arg) noexcept
        {
            if constexpr (RepIdx == TupIdx)
                return std::forward<NewType> (arg);
            else
                return std::get<TupIdx> (tuple);
        }

        template<size_t RepIdx, typename NewType, typename Tuple, size_t... TupIdxs>
        constexpr auto ReplaceTupleElemImpl (Tuple&& tuple, NewType&& arg, std::index_sequence<TupIdxs...>) noexcept
        {
            return std::tuple
            {
                GetReplaceTupleElem<RepIdx, TupIdxs> (std::forward<Tuple> (tuple), std::forward<NewType> (arg))...
            };
        }

        template<size_t RepIdx, typename NewType, typename... TupleArgs>
        constexpr auto ReplaceTupleElem (std::tuple<TupleArgs...>&& tuple, NewType&& arg) noexcept
        {
            return ReplaceTupleElemImpl<RepIdx> (std::move (tuple),
                    std::forward<NewType> (arg),
                    std::index_sequence_for<TupleArgs...> {});
        }

        template<typename Seq, typename T>
        struct DetectShift
        {
            constexpr static int Value = 1;
        };

        template<typename Seq, typename... Args>
        struct DetectShift<Seq, std::tuple<Args...>>
        {
            constexpr static int Value = (DetectShift<Seq, Args>::Value + ...);
        };

        template<typename Seq>
        struct DetectShift<Seq, Seq>
        {
            constexpr static int Value = SeqSize<Seq>;
        };

        template<typename... LArgs, typename... RArgs>
        auto Combine (std::tuple<LArgs...>&& left, std::tuple<RArgs...>&& right) noexcept
        {
            return std::tuple_cat (std::move (left), std::move (right));
        }

        template<typename... LArgs, typename R>
        auto Combine (std::tuple<LArgs...>&& left, const R& right) noexcept
        {
            return std::tuple_cat (std::move (left), std::tuple { right });
        }

        template<typename L, typename... RArgs>
        auto Combine (const L& left, std::tuple<RArgs...>&& right) noexcept
        {
            return std::tuple_cat (std::tuple { left }, std::move (right));
        }

        template<typename L, typename R>
        auto Combine (const L& left, const R& right) noexcept
        {
            return std::tuple { left, right };
        }

        enum class ResultBehaviour
        {
            All,
            First,
        };

        template<ResultBehaviour ResultBehaviour, typename ResList>
        decltype (auto) HandleResultBehaviour (ResList&& list) noexcept
        {
            if constexpr (ResultBehaviour == ResultBehaviour::All)
                return std::forward<ResList> (list);
            else if constexpr (ResultBehaviour == ResultBehaviour::First)
                return list.value (0);
        }

        class SelectWrapperCommon
        {
            const QSqlDatabase DB_;
        public:
            SelectWrapperCommon (const QSqlDatabase& db) noexcept
            : DB_ { db }
            {
            }
        protected:
            auto RunQuery (const QString& queryStr,
                    auto&& binder) const
            {
                QSqlQuery query { DB_ };
                query.prepare (queryStr);
                binder (query);

                if (!query.exec ())
                {
                    qCritical () << "select query execution failed";
                    DBLock::DumpError (query);
                    throw QueryException ("fetch query execution failed");
                }

                return query;
            }
        };

        template<typename L, typename O>
        constexpr auto LimitOffsetToString () noexcept
        {
            if constexpr (std::is_same_v<L, LimitNone>)
            {
                static_assert (std::is_same_v<O, OffsetNone>, "LIMIT-less queries currently cannot have OFFSET");
                return ""_ct;
            }
            else
                return " LIMIT :limit "_ct +
                        [] () constexpr
                        {
                            if constexpr (std::is_same_v<O, OffsetNone>)
                                return ""_ct;
                            else
                                return " OFFSET :offset"_ct;
                        } ();
        }

        template<typename L, typename O>
        void BindLimitOffset (QSqlQuery& query, L limit, O offset) noexcept
        {
            if constexpr (!std::is_same_v<std::decay_t<L>, LimitNone>)
                query.bindValue (":limit", qulonglong { limit.Count });
            if constexpr (!std::is_same_v<std::decay_t<O>, OffsetNone>)
                query.bindValue (":offset", qulonglong { offset.Count });
        }

        template<typename T, typename Selector>
        struct HandleSelector {};

        struct HSBaseAll { constexpr static auto ResultBehaviour_v = ResultBehaviour::All; };
        struct HSBaseFirst { constexpr static auto ResultBehaviour_v = ResultBehaviour::First; };

        template<typename T>
        struct HandleSelector<T, SelectWhole> : HSBaseAll
        {
            constexpr static auto Fields = JoinTup (QualifiedFieldNames<T>, ", "_ct);

            static auto Initializer (const QSqlQuery& q, int startIdx)
            {
                return InitializeFromQuery<T> (q, SeqIndices<T>, startIdx);
            }
        };

        template<typename T, typename U>
        struct HandleSelector<T, SelectDistinct<U>> : HandleSelector<T, U>
        {
            constexpr static auto Fields = "DISTINCT "_ct + HandleSelector<T, U>::Fields;
        };

        template<typename T, auto... Ptrs>
        struct HandleSelector<T, MemberPtrs<Ptrs...>> : HSBaseAll
        {
            constexpr static auto Fields = Join (", ", QualifiedFieldNameByPtr<Ptrs>...);

            static auto Initializer (const QSqlQuery& q, int startIdx) noexcept
            {
                return MakeIndexedQueryHandler (MemberPtrs<Ptrs...> {}, q, startIdx);
            }
        };

        template<typename T, typename FunRetType, CtString Fun, auto Ptr>
        struct HandleSelector<T, CustomFunctionType<FunRetType, Fun, Ptr>> : HSBaseAll
        {
            constexpr static auto Fields = ReplaceAll<Fun, '@'> (QualifiedFieldNameByPtr<Ptr>);

            static auto Initializer (const QSqlQuery& q, int startIdx)
            {
                return Convert<FunRetType> (q.value (startIdx));
            }
        };

        template<typename T>
        struct HandleSelector<T, AggregateType<AggregateFunction::Count, std::true_type {}>> : HSBaseFirst
        {
            constexpr static auto Fields = "count(1)"_ct;

            static auto Initializer (const QSqlQuery& q, int startIdx)
            {
                return q.value (startIdx).toLongLong ();
            }
        };

        template<typename T, auto Ptr>
        struct HandleSelector<T, AggregateType<AggregateFunction::Count, Ptr>> : HSBaseFirst
        {
            constexpr static auto Fields = "count(" + QualifiedFieldNameByPtr<Ptr> + ")";

            static auto Initializer (const QSqlQuery& q, int startIdx)
            {
                return q.value (startIdx).toLongLong ();
            }
        };

        template<CtString Aggregate, typename T, auto Ptr>
        struct HandleAggSelector : HSBaseFirst
        {
            constexpr static auto Fields = Aggregate + "(" + QualifiedFieldNameByPtr<Ptr> + ")";

            static auto Initializer (const QSqlQuery& q, int startIdx) noexcept
            {
                const auto& var = q.value (startIdx);
                return var.isNull () ?
                        std::nullopt :
                        std::optional { Convert<UnwrapIndirect_t<MemberPtrType_t<Ptr>>> (var) };
            }
        };

        template<typename T, auto Ptr>
        struct HandleSelector<T, AggregateType<AggregateFunction::Min, Ptr>> : HandleAggSelector<"min"_ct, T, Ptr> {};

        template<typename T, auto Ptr>
        struct HandleSelector<T, AggregateType<AggregateFunction::Max, Ptr>> : HandleAggSelector<"max"_ct, T, Ptr> {};

        constexpr auto CombineBehaviour (ResultBehaviour l, ResultBehaviour r) noexcept
        {
            if (l == ResultBehaviour::First && r == ResultBehaviour::First)
                return ResultBehaviour::First;
            return ResultBehaviour::All;
        }

        template<typename T, typename L, typename R>
        struct HandleSelector<T, SelectorUnion<L, R>>
        {
            using HL = HandleSelector<T, L>;
            using HR = HandleSelector<T, R>;

            constexpr static auto ResultBehaviour_v = CombineBehaviour (HL::ResultBehaviour_v, HR::ResultBehaviour_v);

            constexpr static auto Fields = HL::Fields + ", " + HR::Fields;

            static auto Initializer (const QSqlQuery& q, int startIdx) noexcept
            {
                constexpr auto shift = DetectShift<T, decltype (HL::Initializer (q, 0))>::Value;
                return Combine (HL::Initializer (q, startIdx), HR::Initializer (q, startIdx + shift));
            }
        };

        template<typename T, SelectBehaviour SelectBehaviour>
        class SelectWrapper : SelectWrapperCommon
        {
            template<typename ParamsTuple>
            struct Builder
            {
                const SelectWrapper& W_;
                ParamsTuple Params_;

                template<typename NewTuple>
                constexpr auto RepTuple (NewTuple&& tuple) noexcept
                {
                    return Builder<NewTuple> { W_, tuple };
                }

                template<typename U>
                constexpr auto Select (U&& selector) && noexcept
                {
                    return RepTuple (ReplaceTupleElem<0> (std::move (Params_), std::forward<U> (selector)));
                }

                template<typename U>
                constexpr auto Where (U&& tree) && noexcept
                {
                    return RepTuple (ReplaceTupleElem<1> (std::move (Params_), std::forward<U> (tree)));
                }

                template<typename U>
                constexpr auto AndWhere (U&& tree) && noexcept
                {
                    return std::move (*this).Where (std::get<1> (Params_) && std::forward<U> (tree));
                }

                template<typename U>
                constexpr auto Order (U&& order) && noexcept
                {
                    return RepTuple (ReplaceTupleElem<2> (std::move (Params_), std::forward<U> (order)));
                }

                template<typename U>
                constexpr auto Group (U&& group) && noexcept
                {
                    return RepTuple (ReplaceTupleElem<3> (std::move (Params_), std::forward<U> (group)));
                }

                constexpr auto Limit (Limit limit) && noexcept
                {
                    return RepTuple (ReplaceTupleElem<4> (std::move (Params_), limit));
                }

                constexpr auto Limit (uint64_t limit) && noexcept
                {
                    return std::move (*this).Limit (oral::Limit { limit });
                }

                constexpr auto Offset (Offset offset) && noexcept
                {
                    return RepTuple (ReplaceTupleElem<5> (std::move (Params_), offset));
                }

                constexpr auto Offset (uint64_t offset) && noexcept
                {
                    return std::move (*this).Offset (oral::Offset { offset });
                }

                auto operator() () &&
                {
                    return std::apply (W_, Params_);
                }

                template<auto... Ptrs>
                constexpr auto Group () && noexcept
                {
                    return std::move (*this).Group (GroupBy<Ptrs...> {});
                }
            };
        public:
            using SelectWrapperCommon::SelectWrapperCommon;

            auto Build () const noexcept
            {
                std::tuple defParams
                {
                    SelectWhole {},
                    ConstTrueTree_v,
                    OrderNone {},
                    GroupNone {},
                    LimitNone {},
                    OffsetNone {}
                };
                return Builder<decltype (defParams)> { *this, defParams };
            }

            auto operator() () const
            {
                return (*this) (SelectWhole {}, ConstTrueTree_v);
            }

            template<typename Single>
            auto operator() (Single&& single) const
            {
                if constexpr (IsExprTree<std::decay_t<Single>>)
                    return (*this) (SelectWhole {}, std::forward<Single> (single));
                else
                    return (*this) (std::forward<Single> (single), ConstTrueTree_v);
            }

            template<
                    typename Selector,
                    ExprType Type, typename L, typename R,
                    typename Order = OrderNone,
                    typename Group = GroupNone,
                    typename Limit = LimitNone,
                    typename Offset = OffsetNone
                >
            auto operator() (Selector,
                    const ExprTree<Type, L, R>& tree,
                    Order order = OrderNone {},
                    Group group = GroupNone {},
                    Limit limit = LimitNone {},
                    Offset offset = OffsetNone {}) const
            {
                using TreeType_t = ExprTree<Type, L, R>;

                constexpr auto where = ExprTreeToSql<"", T, TreeType_t> ();
                constexpr auto wherePrefix = [where]
                {
                    if constexpr (where.IsEmpty ())
                        return " "_ct;
                    else
                        return " WHERE "_ct;
                } ();
                constexpr auto from = BuildFromClause<TreeType_t> ();
                const auto binder = [&] (QSqlQuery& query)
                {
                    BindExprTree<"", T> (tree, query);
                    BindLimitOffset (query, limit, offset);
                };
                using HS = HandleSelector<T, Selector>;

                constexpr auto query = "SELECT " + HS::Fields +
                        " FROM " + from +
                        wherePrefix + where +
                        HandleOrder (std::forward<Order> (order)) +
                        HandleGroup (std::forward<Group> (group)) +
                        LimitOffsetToString<Limit, Offset> ();
                auto selectResult = Select<HS> (ToString<query> (), binder);
                return HandleResultBehaviour<HS::ResultBehaviour_v> (std::move (selectResult));
            }
        private:
            template<typename HS, typename Binder>
            auto Select (const QString& queryStr,
                    Binder&& binder) const
            {
                auto query = RunQuery (queryStr, binder);

                if constexpr (SelectBehaviour == SelectBehaviour::Some)
                {
                    QList<decltype (HS::Initializer (query, 0))> result;
                    while (query.next ())
                        result << HS::Initializer (query, 0);
                    return result;
                }
                else
                {
                    using RetType_t = std::optional<decltype (HS::Initializer (query, 0))>;
                    return query.next () ?
                        RetType_t { HS::Initializer (query, 0) } :
                        RetType_t {};
                }
            }

            template<typename Tree>
            consteval static auto BuildFromClause () noexcept
            {
                if constexpr (Tree::template HasAdditionalTables<T> ())
                    return T::ClassName + ", " + JoinTup (Nub<Tree::template AdditionalTables<T>> (), ", ");
                else
                    return T::ClassName;
            }

            constexpr static auto HandleOrder (OrderNone) noexcept
            {
                return ""_ct;
            }

            template<auto... Ptrs>
            constexpr static auto HandleSuborder (sph::asc<Ptrs...>) noexcept
            {
                return std::tuple { (QualifiedFieldNameByPtr<Ptrs> + " ASC")... };
            }

            template<auto... Ptrs>
            constexpr static auto HandleSuborder (sph::desc<Ptrs...>) noexcept
            {
                return std::tuple { (QualifiedFieldNameByPtr<Ptrs> + " DESC")... };
            }

            template<typename... Suborders>
            constexpr static auto HandleOrder (OrderBy<Suborders...>) noexcept
            {
                return " ORDER BY " + JoinTup (std::tuple_cat (HandleSuborder (Suborders {})...), ", ");
            }

            constexpr static auto HandleGroup (GroupNone) noexcept
            {
                return ""_ct;
            }

            template<auto... Ptrs>
            constexpr static auto HandleGroup (GroupBy<Ptrs...>) noexcept
            {
                return " GROUP BY " + Join (", ", QualifiedFieldNameByPtr<Ptrs>...);
            }
        };

        template<typename T>
        class DeleteByFieldsWrapper
        {
            const QSqlDatabase DB_;
        public:
            DeleteByFieldsWrapper (const QSqlDatabase& db) noexcept
            : DB_ { db }
            {
            }

            template<ExprType Type, typename L, typename R>
            void operator() (const ExprTree<Type, L, R>& tree) const noexcept
            {
                constexpr auto where = ExprTreeToSql<"", T, ExprTree<Type, L, R>> ();

                constexpr auto selectAll = "DELETE FROM " + T::ClassName + " WHERE " + where;

                QSqlQuery query { DB_ };
                query.prepare (ToString<selectAll> ());
                BindExprTree<"", T> (tree, query);
                query.exec ();
            }
        };

        template<typename T>
        class AdaptUpdate
        {
            const QSqlDatabase DB_;

            // TODO this needn't be present of T doesn't have a PKey
            QSqlQuery UpdateByPKey_ { DB_ };
        public:
            AdaptUpdate (const QSqlDatabase& db) noexcept
            : DB_ { db }
            {
                if constexpr (HasPKey<T>)
                {
                    constexpr auto pkeyIdx = PKeyIndex_v<T>;
                    constexpr auto statements = ZipWith (FieldNames<T>, " = ", BoundFieldNames<T>);
                    constexpr auto update = "UPDATE " + T::ClassName +
                            " SET " + JoinTup (statements, ", ") +
                            " WHERE " + std::get<pkeyIdx> (statements);
                    UpdateByPKey_.prepare (ToString<update> ());
                }
            }

            void operator() (const T& seq) requires HasPKey<T>
            {
                DoInsert (seq, UpdateByPKey_, true);
            }

            template<typename SL, typename SR, ExprType WType, typename WL, typename WR>
            int operator() (const AssignList<SL, SR>& set, const ExprTree<WType, WL, WR>& where)
            {
                static_assert (!ExprTree<WType, WL, WR>::template HasAdditionalTables<T> (),
                        "joins in update statements are not supported by SQL");

                constexpr auto setClause = ExprTreeToSql<"set_", T, AssignList<SL, SR>> ();
                constexpr auto whereClause = ExprTreeToSql<"where_", T, ExprTree<WType, WL, WR>> ();

                constexpr auto update = "UPDATE " + T::ClassName +
                        " SET " + setClause +
                        " WHERE " + whereClause;

                QSqlQuery query { DB_ };
                query.prepare (ToString<update> ());
                BindExprTree<"set_", T> (set, query);
                BindExprTree<"where_", T> (where, query);
                if (!query.exec ())
                {
                    qCritical () << "update query execution failed";
                    DBLock::DumpError (query);
                    throw QueryException ("update query execution failed");
                }

                return query.numRowsAffected ();
            }
        };

        template<
                template<typename...> typename Tgt = std::tuple,
                template<typename...> typename Src,
                typename... Vals
            >
        constexpr auto MapTy (Src<Vals...>, auto f)
        {
            return Tgt { f (Vals {})... };
        }

        template<typename Seq, auto... Ptrs>
        constexpr auto CreateIndex (const QSqlDatabase& db, Index<Ptrs...>)
        {
            constexpr auto query = "CREATE INDEX IF NOT EXISTS "_ct +
                    "idx_" + Seq::ClassName + "_" + Join ("_", FieldNameByPtr<Ptrs>...) +
                    " ON " + Seq::ClassName + " (" + Join (", ", FieldNameByPtr<Ptrs>...) + ")";
            RunTextQuery (db, ToString<query> ());
            return Void {};
        }

        template<typename Seq>
        void AdaptCreateIndices (const QSqlDatabase& db)
        {
            if constexpr (requires { typename Seq::Indices; })
                MapTy (typename Seq::Indices {}, [&db] (auto index) { return CreateIndex<Seq> (db, index); });
        }

        template<auto... Ptrs>
        constexpr auto ExtractConstraintFields (UniqueSubset<Ptrs...>)
        {
            return "UNIQUE (" + Join (", ", FieldNameByPtr<Ptrs>...) + ")";
        }

        template<auto... Ptrs>
        constexpr auto ExtractConstraintFields (PrimaryKey<Ptrs...>)
        {
            return "PRIMARY KEY (" + Join (", ", FieldNameByPtr<Ptrs>...) + ")";
        }

        template<typename T>
        constexpr auto GetConstraintsStrings () noexcept
        {
            if constexpr (requires { typename T::Constraints; })
                return MapTy (typename T::Constraints {}, [] (auto ctr) { return ExtractConstraintFields (ctr); });
            else
                return std::tuple {};
        }

        template<typename ImplFactory, typename T, size_t... Indices>
        constexpr auto GetTypes (std::index_sequence<Indices...>) noexcept
        {
            return std::tuple { Type2Name<ImplFactory, ValueAtC_t<T, Indices>> {} ()... };
        }

        template<auto Name, typename ImplFactory, typename T>
        constexpr auto AdaptCreateTableNamed () noexcept
        {
            constexpr auto types = GetTypes<ImplFactory, T> (SeqIndices<T>);

            constexpr auto constraints = GetConstraintsStrings<T> ();
            constexpr auto constraintsStr = [&]
            {
                if constexpr (!std::tuple_size_v<decltype (constraints)>)
                    return ""_ct;
                else
                    return ", " + JoinTup (constraints, ", ");
            } ();

            constexpr auto statements = ZipWith (FieldNames<T>, " ", types);
            return "CREATE TABLE " +
                    Name +
                    " (" +
                    JoinTup (statements, ", ") +
                    constraintsStr +
                    ");";
        }

        template<typename ImplFactory, typename T>
        constexpr auto AdaptCreateTable () noexcept
        {
            return AdaptCreateTableNamed<T::ClassName, ImplFactory, T> ();
        }
    }

    template<typename T>
    struct ObjectInfo
    {
        detail::AdaptInsert<T> Insert;
        detail::AdaptUpdate<T> Update;
        detail::AdaptDelete<T> Delete;

        detail::SelectWrapper<T, detail::SelectBehaviour::Some> Select;
        detail::SelectWrapper<T, detail::SelectBehaviour::One> SelectOne;
        detail::DeleteByFieldsWrapper<T> DeleteBy;

        using ObjectType_t = T;
    };

    template<typename T, typename ImplFactory = detail::SQLite::ImplFactory>
    ObjectInfo<T> Adapt (const QSqlDatabase& db)
    {
        static_assert (BeenAdapted<T>, "Please use ORAL_ADAPT_STRUCT");

        if (!db.tables ().contains (ToString<T::ClassName> (), Qt::CaseInsensitive))
        {
            constexpr auto query = detail::AdaptCreateTable<ImplFactory, T> ();
            RunTextQuery (db, ToString<query> ());
        }

        detail::AdaptCreateIndices<T> (db);

        return
        {
            { db },
            { db },
            { db },

            { db },
            { db },
            { db },
        };
    }

    template<typename T>
    using ObjectInfo_ptr = std::unique_ptr<ObjectInfo<T>>;

    template<typename T, typename ImplFactory = SQLiteImplFactory>
    ObjectInfo_ptr<T> AdaptPtr (const QSqlDatabase& db)
    {
        return std::make_unique<ObjectInfo<T>> (Adapt<T, ImplFactory> (db));
    }

    template<typename ImplFactory = SQLiteImplFactory, typename... Ts>
    void AdaptPtrs (const QSqlDatabase& db, ObjectInfo_ptr<Ts>&... objects)
    {
        ((objects = AdaptPtr<Ts, ImplFactory> (db)), ...);
    }
}