Question

I’m trying to use synonyms in a mix in for a declarative class model.

class MyMixin(object):

    __my_field = Column(Boolean, name='my_field', index=True, default=True)

    def __get_my_field(self):
        return self.__my_field

    @declared_attr
    def my_field(cls):  # @NoSelf
        return synonym('__my_field', descriptor=property(cls.__get_my_field))    


Base = declarative_base(cls=MyMixin)


class Model(Base):

    __tablename__ = 'model'

    value = Column(String)

The code starts up fine, but whenever I try to query on that field (session.query(Model).filter(Model.my_field==True)), I get a maximum recursion error.

I’ve tried the solution suggested in this other question, but all I get is a Maximum Recursion Exceeded error.

Answer 1

Python applies name mangling to class attributes that start with a double underscore, and I strongly suspect that that is the root of your problem here.

Remove one of the underscores from your column declaration:

class MyMixin(object):

    __my_field = Column(Boolean, name='my_field', index=True, default=True)

    def _get_my_field(self):
        return self._my_field

    @declared_attr
    def my_field(cls):  # @NoSelf
        return synonym('_my_field', descriptor=property(cls._get_my_field))