[Sqlalchemy-tickets] Issue #4142: SQLAlchemy accesses attribute table of custom ColumnElement (zzzeek/sqlalchemy)

[dima-starosud <iss...@bi...>]

New issue 4142: SQLAlchemy accesses attribute table of custom ColumnElement
https://bitbucket.org/zzzeek/sqlalchemy/issues/4142/sqlalchemy-accesses-attribute-table-of

dima-starosud:

This is using sqlalchemy 1.2.0b3.

Please considering following code snippet. It fails with an error (shown in code). However there is a workaround to just add `table = None` to `my_literal`, but I suppose this may lead to other problems.

```
#!python

        Base = declarative_base()

        class my_literal(expression.ColumnElement):
            type = Unicode
            # table = None  # oO

        @compiles(my_literal)
        def default_my_literal(element, compiler, **kw):
            return "'my literal'"

        class _A(Base):
            __tablename__ = 'a_table'
            id = Column(Integer, primary_key=True)
            ml = column_property(my_literal())
            __mapper_args__ = {'polymorphic_on': ml}

        class _B(Base):
            __tablename__ = 'b_table'
            id = Column(Integer, primary_key=True)
            a_id = Column(Integer, ForeignKey(_A.id))
            ml = column_property(my_literal())
            __mapper_args__ = {'polymorphic_on': ml}

        class _C(Base):
            __tablename__ = 'c_table'
            id = Column(Integer, primary_key=True)
            b_id = Column(Integer, ForeignKey(_B.id))
            ml = column_property(my_literal())
            __mapper_args__ = {'polymorphic_on': ml}

        class A(_A):
            __mapper_args__ = {'polymorphic_identity': 'my literal'}

        class B(_B):
            __mapper_args__ = {'polymorphic_identity': 'my literal'}

        class C(_C):
            __mapper_args__ = {'polymorphic_identity': 'my literal'}

        B.a = relationship(A, backref='bs')
        C.b = relationship(B, backref='cs')
        A.cs = association_proxy('bs', 'cs')

        Base.metadata.create_all(bind=session.connection())

        a = A(bs=[B(cs=[C(), C()]), B(cs=[C(), C()])])
        session.add(a)
        session.flush()
        session.expire_all()
        assert a.cs  # AttributeError: Neither 'my_literal' object
                     # nor 'Comparator' object has an attribute 'table'

```