Nested single string 'only' behavior edge case

When using Nested fields, passing a single string field to only does not work as expected when applying a transform with on_bind_field

from marshmallow import fields, Schema


def test_nested_only_on_bind_field():

    def to_camel_case(snake_str):
        components = snake_str.split('_')
        return components[0] + ''.join(x.title() for x in components[1:])

    class CamelCaseSchema(Schema):
        def on_bind_field(self, field_name, field_obj):
            field_obj.data_key = to_camel_case(field_name)

    class UserRoleSchema(CamelCaseSchema):
        user_id = fields.Number()
        role_name = fields.String()

    class UserSchema(CamelCaseSchema):
        id = fields.Number()
        roles = fields.Nested(UserRoleSchema, many=True, only='role_name')

    user = {
        'id': 1,
        'roles': [
            {'user_id': 1, 'role_name': 'admin'},
            {'user_id': 1, 'role_name': 'test'},
        ]
    }

    serialized_user = UserSchema().dumps(user)

    assert serialized_user == {"id": 1, "roles": ["admin", "test"]}

This results in the following error:

/env/lib/python3.6/site-packages/marshmallow/schema.py:476: in dumps
    serialized = self.dump(obj, many=many, update_fields=update_fields)
/env/lib/python3.6/site-packages/marshmallow/schema.py:428: in dump
    **kwargs
/env/lib/python3.6/site-packages/marshmallow/marshalling.py:147: in serialize
    index=(index if index_errors else None)
/env/lib/python3.6/site-packages/marshmallow/marshalling.py:68: in call_and_store
    value = getter_func(data)
/env/lib/python3.6/site-packages/marshmallow/marshalling.py:141: in <lambda>
    getter = lambda d: field_obj.serialize(attr_name, d, accessor=accessor)
/env/lib/python3.6/site-packages/marshmallow/fields.py:250: in serialize
    return self._serialize(value, attr, obj)
/env/lib/python3.6/site-packages/marshmallow/fields.py:455: in _serialize
    return utils.pluck(ret, key=self.only)
/env/lib/python3.6/site-packages/marshmallow/utils.py:318: in pluck
    return [d[key] for d in dictlist]
KeyError: 'role_name'

This seems to be only affecting the single string case, as passing only=['role_name'] works as expected.

[deckar01]

Would it make sense to remove this support entirely for v3?

I think modulating the Nested functionality this much based on the type of a parameter is confusing. The feature is useful though.

What about a dedicated field type to encapsulate this behavior?

class UserSchema(Schema):
    id = fields.Number()
    roles = fields.Pluck(UserRoleSchema, 'role_name', many=True)

I think that interface would communicate the behavior without needing to dig into the docs.

[taion]

As a side note, if we go ahead with https://github.com/marshmallow-code/marshmallow/pull/810, then __filter_fields is gone anyway. Though I agree that fields.Pluck makes more sense regardless.