Keeping form together with models is breaking SQLAlchemy on relationships

[hyperknot]

Basically, having model definitions like this:

init.py

BaseModelForm = model_form_factory(ModelForm_)


class ModelForm(BaseModelForm):
    class Meta:
        include_primary_keys = True
        include_foreign_keys = True

    @classmethod
    def get_session(self): # noqa
        return DBSession

parent.py

from sqlalchemy import Column, Integer
from sqlalchemy.orm import relationship

from . import Base, ModelForm

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", back_populates="parent")

class MapForm(ModelForm):
    class Meta:
        model = Parent

child.py

from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship

from . import Base


class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", back_populates="children")
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It breaks initializing with the following error message:

  File "/Users/user/.virtualenvs/temp/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1601, in mapper
    argument = self.argument()
  File "/Users/user/.virtualenvs/temp/lib/python2.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 293, in __call__
    (self.prop.parent, self.arg, n.args[0], self.cls)
sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper|Parent|parent, expression 'Child' failed to locate a name ("name 'Child' is not defined"). If this is a class name, consider adding this relationship() to the <class 'pyramidsqlalchemyrelationshipbug.models.parent.Parent'> class after both dependent classes have been defined.

I made a repo out of a minimal project:
https://github.com/hyperknot/pyramid-sqlalchemy-wtforms-relationship-bug