SQLAlchemy accesses attribute table of custom ColumnElement

[sqlalchemy-bot]

Migrated issue, originally created by Dmytro Starosud (@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.

        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'

Stack trace.

/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/ext/associationproxy.py:264: in __get__
    proxy = self._new(_lazy_collection(obj, self.target_collection))
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/ext/associationproxy.py:309: in _new
    self.collection_class = util.duck_type_collection(lazy_collection())
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/ext/associationproxy.py:499: in __call__
    return getattr(obj, self.target)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/attributes.py:242: in __get__
    return self.impl.get(instance_state(instance), dict_)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/attributes.py:603: in get
    value = self.callable_(state, passive)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/strategies.py:623: in _load_for_state
    return self._emit_lazyload(session, state, ident_key, passive)
<string>:1: in <lambda>
    ???
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/strategies.py:733: in _emit_lazyload
    lazy_clause, params = self._generate_lazy_clause(state, passive)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/strategies.py:556: in _generate_lazy_clause
    state, dict_, ident, passive)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py:2609: in _get_state_attr_by_column
    return state.manager[prop.key].impl.get(state, dict_, passive=passive)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/attributes.py:598: in get
    value = state._load_expired(state, passive)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/state.py:594: in _load_expired
    self.manager.deferred_scalar_loader(self, toload)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/loading.py:792: in load_scalar_attributes
    statement = mapper._optimized_get_statement(state, attribute_names)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py:2647: in _optimized_get_statement
    for key in attribute_names
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py:2648: in <listcomp>
    for c in props[key].columns]
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/sql/util.py:148: in find_tables
    visitors.traverse(clause, {'column_collections': False}, _visitors)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py:269: in traverse
    return traverse_using(iterate(obj, opts), obj, visitors)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py:260: in traverse_using
    meth(target)
/home/vagrant/venv/lib/python3.5/site-packages/sqlalchemy/sql/util.py:143: in visit_column
    tables.append(column.table)

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

stack trace.....i have to adapt this to real code and run it to see b.c. not provided...

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

anyway, the answer for now is, put table=None, there's no side effect from that.

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

https://gerrit.sqlalchemy.org/#/c/zzzeek/sqlalchemy/+/610/ will test feasibility of a change to prevent user-defined columnelements from being assumed to be table-bound

[sqlalchemy-bot]

Changes by Michael Bayer (@zzzeek):

• added labels: sql

[sqlalchemy-bot]

Changes by Michael Bayer (@zzzeek):

• set milestone to "1.2"

[sqlalchemy-bot]

Dmytro Starosud (@dima-starosud) wrote:

stack trace

@zzzeek Sorry, will attach later.

[sqlalchemy-bot]

Changes by Dmytro Starosud (@dima-starosud):

• edited description

[sqlalchemy-bot]

Dmytro Starosud (@dima-starosud) wrote:

stack trace

@zzzeek Done.

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

Change visit name for ColumnElement

No SQLA built-in subclasses ColumnElement without specifying
an alternate visit_name, and user defined ColumnElement
subclasses should avoid being treated like ColumnClause,
e.g. where a Table is present.

Fixes: #4142
Change-Id: I15ed09ba8bdebae4cb0c7e5e5df3f59351477577

→ 5e8396a

[sqlalchemy-bot]

Changes by Michael Bayer (@zzzeek):

• changed status to closed

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

my point was, put the stack trace when you first post the issue, so that I can identify the problem without actually having to generate a complete test and run it. just saves me five minutes, that's all.