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Mike Bayer
e07130c597
Added a new parameter to :class:`_orm.Mapper` called :paramref:`_orm.Mapper.polymorphic_abstract`. The purpose of this directive is so that the ORM will not consider the class to be instantiated or loaded directly, only subclasses. The actual effect is that the :class:`_orm.Mapper` will prevent direct instantiation of instances of the class and will expect that the class does not have a distinct polymorphic identity configured. In practice, the class that is mapped with :paramref:`_orm.Mapper.polymorphic_abstract` can be used as the target of a :func:`_orm.relationship` as well as be used in queries; subclasses must of course include polymorphic identities in their mappings. The new parameter is automatically applied to classes that subclass the :class:`.AbstractConcreteBase` class, as this class is not intended to be instantiated. Additionally, updated some areas of the single table inheritance documentation to include mapped_column(nullable=False) for all subclass-only columns; the mappings as given didn't work as the columns were no longer nullable using Annotated Declarative Table style. Fixes: #9060 Change-Id: Ief0278e3945a33a6ff38ac14d39c38ce24910d7f