[Sqlalchemy-tickets] Issue #3468: Deferred column_property and subclassing (zzzeek/sqlalchemy)

[harut <iss...@bi...>]

New issue 3468: Deferred column_property and subclassing
https://bitbucket.org/zzzeek/sqlalchemy/issue/3468/deferred-column_property-and-subclassing

harut:

Hi! There is a quite specific test case for sqlalchemy 1.0.*. Deferred column_property if defined in a subclass, fails after flush.

The test passes in sqlalchemy 0.9.7 and fails in 1.0.0-1.0.9


        class A(self.Base):
            id = Column(Integer, primary_key=True)
        class B(A):
            id = Column(Integer, ForeignKey(A.id), primary_key=True)
            data = Column(String)
            expr = column_property(data+' '+data, deferred=True)

        b = B(id=2, data='aaa')
        db.add(b)
        db.flush()
        b.expr

Gives a following exception:

      File "sqlalchemy/orm/attributes.py", line 237, in __get__
        return self.impl.get(instance_state(instance), dict_)
      File "sqlalchemy/orm/attributes.py", line 592, in get
        "correctly" % key)
    KeyError: "Deferred loader for attribute 'data' failed to populate correctly"

I have no idea what is going on, but the code of `get` function is a little bit strange for me:

    def get(self, state, dict_, passive=PASSIVE_OFF):
        """..."""
        if self.key in dict_:
            return dict_[self.key]
        else:
            key = self.key
            if key not in state.committed_state or \
                    state.committed_state[key] is NEVER_SET:

                ....

                elif value is ATTR_WAS_SET:
                    try:
                        # XXX dict_[key] seems to be always not defined, because we have checked
                        #          self.key in dict_ before
                        return dict_[key]
                    except KeyError:
                        # TODO: no test coverage here.
                        raise KeyError(
                            "Deferred loader for attribute "
                            "%r failed to populate "
                            "correctly" % key)
    ...