Validation not properly applied for mutliply inherited StoredObject subclass instances

[lyndsysimon]

The following code, when executed, runs validation on only two of the three fields upon save:

from modularodm import StoredObject, fields, storage

db = storage.EphemeralStorage()


def mock_validator(*args, **kwargs):
    print("VALIDATING")


class BaseClass(StoredObject):
    _meta = {
        'abstract': True
    }
    pk = fields.IntegerField(primary=True, validate=mock_validator)


class MyMixin(StoredObject):
    _meta = {
        'abstract': True,
    }
    mixed_field = fields.IntegerField(validate=mock_validator)


class DerivedClass(BaseClass, MyMixin):
    child_field = fields.IntegerField(validate=mock_validator)


BaseClass.set_storage(db)
DerivedClass.set_storage(db)

d = DerivedClass(
    pk=1,
    child_field=2,
    mixed_field=3
)
d.save()

I've traced it down to line 800 of storedobject.py, before needing to move on to another task. The getattr call seems to be returning None for fields that were defined in the mixin.