Self-referential nested fields not parsed correctly

[whoiswes]

Apologies if I'm missing something obvious here as I'm still figuring out Marshmallow and APISpec.

When we have a Model with a self-referential relationship, apispec is not able to parse the child relationships successfully. Here's an example (SQLAlchemy, Marshmallow-SQLAlchemy, APISpec, and Flask)

Models:

Parent_Child = Table('Parent_Child', Base.metadata,
    Column('parent_id', Integer, ForeignKey('Entity.id')),
    Column('child_id', Integer, ForeignKey('Entity.id'))
    )

class Entity(Base):
    __tablename__ = 'Entity'
    id = Column(Integer, primary_key=True)
    name = Column(String(80))
    children = relationship('Entity', secondary='Parent_Child', primaryjoin=(id == Parent_Child.c.parent_id), secondaryjoin=(id == Parent_Child.c.child_id), lazy='joined', backref='parent')

Schema:

class EntitySchema(ModelSchema):
    children = fields.Nested('self', many=True)
    class Meta:
        model = Entity
        sqla_session = session
        include_fk = True

EntitySerializer = EntitySchema()

APISpec to_dict output (note the Entity.children object reference)

{
    'parameters': {},
    'paths': {
        '/entities': {}
    },
    'swagger': '2.0',
    'info': {
        'version': '1.0.0',
        'title': 'M2M Nested Example'
    },
    'description': 'M2M Nested Example',
    'definitions': {
        'Entity': {
            'properties': {
                'children': .at 0x00000000048BF7B8 > ,
                'id': {
                    'format': 'int32',
                    'type': 'integer'
                },
                'parent': {
                    'type': 'array',
                    'items': {
                        'type': 'string'
                    }
                },
                'name': {
                    'type': 'string'
                }
            }
        }
    }
}

Again, apologies if I've missed something obvious or this is a known issue. I can provide a complete sample app via gist if that would be helpful.

Thanks!

[whoiswes]

FYI, by accident I got this working. If you change 'self' to the name of the schema you're self-referencing, APISpec doesn't blow up.

Like so:

class EntitySchema(ModelSchema):
    children = fields.Nested('EntitySchema', many=True)
    class Meta:
        model = Entity
        sqla_session = session
        include_fk = True

Not sure if this would be expected or desired behavior so leaving this open for now.

[sloria]

Thanks for the update, @whoiswes . I'm glad you found a workaround, but this still looks to be a bug--passing 'self' should not break the code.

I would be open to a PR for this.

[sloria]

This bug is now fixed on dev.

[Chris2048]

Hi, Can you explain to me the part of this fix:

is_unbound_self_referencing = not getattr(field, 'parent', None) and field.nested == 'self'

How might a self-referencing Nested field be 'bounded'?
Is the test for a parent is the same as a test for being an instance of SchemaABC (top-level abs baseclass)?

I am getting a MaxRecursion error on a self-referencing Nested field. This is fixed by adding a ref, but I'm curious why it is not USR without - the test for there being no parent fails, so the code here doesn't seem to handle.

Regards.

[Chris2048]

oooh, I think I see now - it if you supply a definition via:

spec.definition('foo', schema=FooSchema)

It will then be a "top-level" object since it has its own definition.
Then the name ('foo') is used to fill in the ref.

Is it possible to raise an exception when the object is self-referential but not top-level?
i.e."Self-referencing schema must provide named definition".

Incidentally, what happens when there are Schema that don't involve self-reference, but are recursive, e.g. involving two elements that refer to each other, A -> B -> A -> B -> A -> ...

If neither provides a definition then the same MaxRecursion will occur, without a Nested 'self'?

Regards.