I wrestled through getting a model heirarchy to work with single-table inheritance that is polymorphic on a hybrid attribute on this mailing list a while ago.
see: https://groups.google.com/d/topic/sqlalchemy/KJXSHwbhbLA/discussion The problem I'm running into now is that it doesn't seem to work correctly when I want to use the hybrid property for filtering or ordering. This seems to be an issue with auto-correlation, but I can't seem to figure out how to get it working. Here is a runnable example: from sqlalchemy import * from sqlalchemy.orm import * from sqlalchemy.ext.declarative import declarative_base, declared_attr from sqlalchemy.ext.hybrid import hybrid_property, Comparator Base = declarative_base() class classproperty(property): """A decorator that behaves like @property except that operates on classes rather than instances. The decorator is currently special when using the declarative module, but note that the :class:`~.sqlalchemy.ext.declarative.declared_attr` decorator should be used for this purpose with declarative. """ def __init__(self, fget, *arg, **kw): super(classproperty, self).__init__(fget, *arg, **kw) self.__doc__ = fget.__doc__ def __get__(desc, self, cls): return desc.fget(cls) class ModelBase(Base): __abstract__ = True def __repr__(self): return "{} ({}:{})".format(type(self).__name__, self.name, self.id) class HybridType(ModelBase): __tablename__ = 'hybrid_types' id = Column(Integer, primary_key=True) name = Column(Text) class HybridModel(ModelBase): __tablename__ = 'hybrids' id = Column(Integer, primary_key=True) name = Column(Text) hybrid_type_id = Column(Integer, ForeignKey('hybrid_types.id'), nullable=False) hybrid_type = relationship('HybridType') def __init__(self, *args, **kwargs): self.hybrid_type_name = self.hybrid_type_identity return super().__init__(*args, **kwargs) @classproperty def hybrid_type_identity(cls): return cls.__name__ @declared_attr def __mapper_args__(cls): return dict( polymorphic_on=cls.hybrid_type_name_subquery(), polymorphic_identity=cls.hybrid_type_identity, ) @hybrid_property def hybrid_type_name(self): return self.hybrid_type.name @hybrid_type_name.setter def hybrid_type_name(self, value): self.hybrid_type_id = ( select([HybridType.id]). where(HybridType.name == value) ) @hybrid_type_name.expression def hybrid_type_name(cls): return cls.hybrid_type_name_subquery() @classmethod def hybrid_type_name_subquery(cls): return select([HybridType.name]).where(HybridType.id == cls.hybrid_type_id).as_scalar() class HybridComparator(Comparator): def operate(self, op, other): return op(HybridType.id, select([HybridType.id]).where(HybridType.name == other).as_scalar()) @hybrid_type_name.comparator def hybrid_type_name(cls): return cls.HybridComparator(cls) class HybridAlpha(HybridModel): pass class HybridBeta(HybridModel): pass e = create_engine("sqlite://", echo=False) Base.metadata.create_all(e) session = Session(e) session.add(HybridType(name=HybridAlpha.hybrid_type_identity)) session.add(HybridType(name=HybridBeta.hybrid_type_identity)) session.add(HybridAlpha(name='alpha_instance')) session.add(HybridBeta(name='beta_instance')) print("--- Test query from base hybrid model ---") assert session.query(HybridModel).count() == 2 print("passed") print("--- Test query from base derived hybrid model ---") assert session.query(HybridAlpha).count() == 1 assert session.query(HybridBeta).count() == 1 print("passed") print("--- Test query order_by on hybrid attribute ---") assert [ x.hybrid_type_name for x in session.query(HybridModel).order_by(HybridModel.hybrid_type_name) ] == [HybridAlpha.hybrid_type_identity, HybridBeta.hybrid_type_identity] print("passed") print("--- Test query filter_by on hybrid attribute ---") assert session.query(HybridModel).filter_by(hybrid_type_name=HybridAlpha.hybrid_type_identity).count() == 1 print("passed") print("--- Test query filtered on hybrid attribute ---") assert session.query(HybridModel).filter(HybridAlpha.hybrid_type_name == HybridAlpha.hybrid_type_identity).count() == 1 print("passed") Running this results in the following out put: $ python demo.py --- Test query from base hybrid model --- passed --- Test query from base derived hybrid model --- passed --- Test query order_by on hybrid attribute --- Traceback (most recent call last): File "demo.py", line 121, in <module> in session.query(HybridModel).order_by(HybridModel.hybrid_type_name) File "<string>", line 2, in order_by File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/base.py", line 201, in generate fn(self, *args[1:], **kw) File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 1589, in order_by criterion = self._adapt_col_list(criterion) File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 256, in _adapt_col_list for o in cols File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 256, in <listcomp> for o in cols File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 4191, in _literal_as_label_reference return _literal_as_text(element) File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 4230, in _literal_as_text "instead" % type(element) sqlalchemy.exc.ArgumentError: SQL expression object or string expected, got object of type <class 'sqlalchemy.ext.declarative.api.DeclarativeMeta'> instead -- SQLAlchemy - The Python SQL Toolkit and Object Relational Mapper http://www.sqlalchemy.org/ To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. 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