exception when overwriting columns with two levels of inheritance (3.1.0, 4.0.0a1)

[numberoverzero]

reproducible on both 3.1.0 and 4.0.0a1 with:

#!/usr/bin/env python
from bloop import BaseModel, Column, Integer

class A(BaseModel):
    id_a = Column(Integer, hash_key=True)

class B(A):
    id_b = Column(Integer, hash_key=True)

# B still has a ref to B.id_a
assert all(x in dir(B) for x in {"id_a", "id_b"})
# note that this is the **actual parent id_a**  and not the copy that was temporarily
# created during BaseModel.__init_subclass__
assert B.id_a is A.id_a

# this will fail
class C(B):
    other_field = Column(Integer)

The grandparent is defined correctly, and the parent has the correct hash key (in Meta.keys, Meta.hash_key, Meta.projection, Meta.columns).

However, there are two problems:

1. B.id_a still exists even though it's not actually a key on the model. This isn't the original parent value but a copy that was made, briefly bound during __init_subclass__ and then unbound.
2. When the grandchild class C is being configured during BaseModel.__init_subclass__ the derived_attrs are defined as "all columns or indexes found with inspect.getmembers that are not also found in cls.__dict__" and this contains both the immediate parents' id_b as well as the grandparents' id_a. Since these are both hash keys, the derived_hash_keys check fails.

[numberoverzero]

error message for anyone searching:

bloop.exceptions.InvalidModel: The model C subclasses one or more models that declare multiple columns as the hash key: ['id_a', 'id_b']