Question

I would like to have a ‘relationship’ in an inherited (mixin) class.

However, when I create the inherited object, the relationship object is None. I cannot append to it.

How do I resolve this?

Here is code based upon the documentation

from sqlalchemy import Column, Integer, String, DateTime, Boolean, BigInteger, Float
from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship, backref
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
class Target(Base):
    __tablename__ = "target"
    id = Column(Integer, primary_key=True)

class RefTargetMixin(object):
    @declared_attr
    def target_id(cls):
        return Column('target_id', ForeignKey('target.id'))

    @declared_attr
    def target(cls):
        return relationship("Target",
            primaryjoin="Target.id==%s.target_id" % cls.__name__
        )

class Foo(RefTargetMixin, Base):
    __tablename__ = 'foo'
    id = Column(Integer, primary_key=True)

print repr(RefTargetMixin.target)
print repr(Foo.target)
print repr(Foo().target)

The output is:

<sqlalchemy.orm.properties.RelationshipProperty object at 0x24e7890>
<sqlalchemy.orm.attributes.InstrumentedAttribute object at 0x24e7690>
None

In general, I should be able to append to the relationship object (target), but here I cannot because it is None. Why?

Answer 1

This is the normal behaviour : Foo has one Target. When you create the Foo object, it has no Target yet, so the value of Foo().target is None.

If you want Foo to have multiple Targets, you should put a foo_id in Target, and not a target_id in Foo, and use a backref.

Also, in that case, it is not needed to specify the primary join.