Release: 2.1.0b1 | Release Date: January 21, 2026

# SQLAlchemy 2.1 Documentation

# Source code for examples.versioned_history.history_meta

    """Versioned mixin class and other utilities."""

    import datetime

    from sqlalchemy import and_
    from sqlalchemy import Column
    from sqlalchemy import DateTime
    from sqlalchemy import event
    from sqlalchemy import ForeignKeyConstraint
    from sqlalchemy import func
    from sqlalchemy import inspect
    from sqlalchemy import Integer
    from sqlalchemy import PrimaryKeyConstraint
    from sqlalchemy import select
    from sqlalchemy import util
    from sqlalchemy.orm import attributes
    from sqlalchemy.orm import object_mapper
    from sqlalchemy.orm.exc import UnmappedColumnError
    from sqlalchemy.orm.relationships import RelationshipProperty

    def col_references_table(col, table):
        for fk in col.foreign_keys:
            if fk.references(table):
                return True
        return False

    def _is_versioning_col(col):
        return "version_meta" in col.info

    def _history_mapper(local_mapper):
        cls = local_mapper.class_

        if cls.__dict__.get("_history_mapper_configured", False):
            return

        cls._history_mapper_configured = True

        super_mapper = local_mapper.inherits
        polymorphic_on = None
        super_fks = []
        properties = util.OrderedDict()

        if super_mapper:
            super_history_mapper = super_mapper.class_.__history_mapper__
        else:
            super_history_mapper = None

        if (
            not super_mapper
            or local_mapper.local_table is not super_mapper.local_table
        ):
            version_meta = {"version_meta": True}  # add column.info to identify
            # columns specific to versioning

            history_table = local_mapper.local_table.to_metadata(
                local_mapper.local_table.metadata,
                name=local_mapper.local_table.name + "_history",
            )
            for idx in history_table.indexes:
                if idx.name is not None:
                    idx.name += "_history"
                idx.unique = False

            for orig_c, history_c in zip(
                local_mapper.local_table.c, history_table.c
            ):
                orig_c.info["history_copy"] = history_c
                history_c.unique = False
                history_c.default = history_c.server_default = None
                history_c.autoincrement = False

                if super_mapper and col_references_table(
                    orig_c, super_mapper.local_table
                ):
                    assert super_history_mapper is not None
                    super_fks.append(
                        (
                            history_c.key,
                            list(super_history_mapper.local_table.primary_key)[0],
                        )
                    )
                if orig_c is local_mapper.polymorphic_on:
                    polymorphic_on = history_c

                orig_prop = local_mapper.get_property_by_column(orig_c)
                # carry over column re-mappings
                if (
                    len(orig_prop.columns) > 1
                    or orig_prop.columns[0].key != orig_prop.key
                ):
                    properties[orig_prop.key] = tuple(
                        col.info["history_copy"] for col in orig_prop.columns
                    )

            for const in list(history_table.constraints):
                if not isinstance(
                    const, (PrimaryKeyConstraint, ForeignKeyConstraint)
                ):
                    history_table.constraints.discard(const)

            # "version" stores the integer version id.  This column is
            # required.
            history_table.append_column(
                Column(
                    "version",
                    Integer,
                    primary_key=True,
                    autoincrement=False,
                    info=version_meta,
                )
            )

            # "changed" column stores the UTC timestamp of when the
            # history row was created.
            # This column is optional and can be omitted.
            history_table.append_column(
                Column(
                    "changed",
                    DateTime,
                    default=lambda: datetime.datetime.now(datetime.timezone.utc),
                    info=version_meta,
                )
            )

            if super_mapper:
                super_fks.append(
                    ("version", super_history_mapper.local_table.c.version)
                )

            if super_fks:
                history_table.append_constraint(
                    ForeignKeyConstraint(*zip(*super_fks))
                )

        else:
            history_table = None
            super_history_table = super_mapper.local_table.metadata.tables[
                super_mapper.local_table.name + "_history"
            ]

            # single table inheritance.  take any additional columns that may have
            # been added and add them to the history table.
            for column in local_mapper.local_table.c:
                if column.key not in super_history_table.c:
                    col = Column(
                        column.name, column.type, nullable=column.nullable
                    )
                    super_history_table.append_column(col)

        if not super_mapper:

            def default_version_from_history(context):
                # Set default value of version column to the maximum of the
                # version in history columns already present +1
                # Otherwise re-appearance of deleted rows would cause an error
                # with the next update
                current_parameters = context.get_current_parameters()
                return context.connection.scalar(
                    select(
                        func.coalesce(func.max(history_table.c.version), 0) + 1
                    ).where(
                        and_(
                            *[
                                history_table.c[c.name]
                                == current_parameters.get(c.name, None)
                                for c in inspect(
                                    local_mapper.local_table
                                ).primary_key
                            ]
                        )
                    )
                )

            local_mapper.local_table.append_column(
                Column(
                    "version",
                    Integer,
                    # if rows are not being deleted from the main table with
                    # subsequent reuse of primary key, this default can be
                    # "1" instead of running a query per INSERT
                    default=default_version_from_history,
                    nullable=False,
                ),
                replace_existing=True,
            )
            local_mapper.add_property(
                "version", local_mapper.local_table.c.version
            )

            if cls.use_mapper_versioning:
                local_mapper.version_id_col = local_mapper.local_table.c.version

        # set the "active_history" flag
        # on on column-mapped attributes so that the old version
        # of the info is always loaded (currently sets it on all attributes)
        for prop in local_mapper.iterate_properties:
            prop.active_history = True

        super_mapper = local_mapper.inherits

        if super_history_mapper:
            bases = (super_history_mapper.class_,)

            if history_table is not None:
                properties["changed"] = (history_table.c.changed,) + tuple(
                    super_history_mapper.attrs.changed.columns
                )

        else:
            bases = local_mapper.base_mapper.class_.__bases__

        versioned_cls = type(
            "%sHistory" % cls.__name__,
            bases,
            {
                "_history_mapper_configured": True,
                "__table__": history_table,
                "__mapper_args__": dict(
                    inherits=super_history_mapper,
                    polymorphic_identity=local_mapper.polymorphic_identity,
                    polymorphic_on=polymorphic_on,
                    properties=properties,
                ),
            },
        )

        cls.__history_mapper__ = versioned_cls.__mapper__

    class Versioned:
        use_mapper_versioning = False
        """if True, also assign the version column to be tracked by the mapper"""

        __table_args__ = {"sqlite_autoincrement": True}
        """Use sqlite_autoincrement, to ensure unique integer values
        are used for new rows even for rows that have been deleted."""

        def __init_subclass__(cls) -> None:
            insp = inspect(cls, raiseerr=False)

            if insp is not None:
                _history_mapper(insp)
            else:

                @event.listens_for(cls, "after_mapper_constructed")
                def _mapper_constructed(mapper, class_):
                    _history_mapper(mapper)

            super().__init_subclass__()

    def versioned_objects(iter_):
        for obj in iter_:
            if hasattr(obj, "__history_mapper__"):
                yield obj

    def create_version(obj, session, deleted=False):
        obj_mapper = object_mapper(obj)
        history_mapper = obj.__history_mapper__
        history_cls = history_mapper.class_

        obj_state = attributes.instance_state(obj)

        attr = {}

        obj_changed = False

        for om, hm in zip(
            obj_mapper.iterate_to_root(), history_mapper.iterate_to_root()
        ):
            if hm.single:
                continue

            for hist_col in hm.local_table.c:
                if _is_versioning_col(hist_col):
                    continue

                obj_col = om.local_table.c[hist_col.key]

                # get the value of the
                # attribute based on the MapperProperty related to the
                # mapped column.  this will allow usage of MapperProperties
                # that have a different keyname than that of the mapped column.
                try:
                    prop = obj_mapper.get_property_by_column(obj_col)
                except UnmappedColumnError:
                    # in the case of single table inheritance, there may be
                    # columns on the mapped table intended for the subclass only.
                    # the "unmapped" status of the subclass column on the
                    # base class is a feature of the declarative module.
                    continue

                # expired object attributes and also deferred cols might not
                # be in the dict.  force it to load no matter what by
                # using getattr().
                if prop.key not in obj_state.dict:
                    getattr(obj, prop.key)

                a, u, d = attributes.get_history(obj, prop.key)

                if d:
                    attr[prop.key] = d[0]
                    obj_changed = True
                elif u:
                    attr[prop.key] = u[0]
                elif a:
                    # if the attribute had no value.
                    attr[prop.key] = a[0]
                    obj_changed = True

        if not obj_changed:
            # not changed, but we have relationships.  OK
            # check those too
            for prop in obj_mapper.iterate_properties:
                if (
                    isinstance(prop, RelationshipProperty)
                    and attributes.get_history(
                        obj, prop.key, passive=attributes.PASSIVE_NO_INITIALIZE
                    ).has_changes()
                ):
                    for p in prop.local_columns:
                        if p.foreign_keys:
                            obj_changed = True
                            break
                    if obj_changed is True:
                        break

        if not obj_changed and not deleted:
            return

        attr["version"] = obj.version
        hist = history_cls()
        for key, value in attr.items():
            setattr(hist, key, value)
        session.add(hist)
        obj.version += 1

    def versioned_session(session):
        @event.listens_for(session, "before_flush")
        def before_flush(session, flush_context, instances):
            for obj in versioned_objects(session.dirty):
                create_version(obj, session)
            for obj in versioned_objects(session.deleted):
                create_version(obj, session, deleted=True)