fields.Nested should optionally pass the parent object to the Nested Schema

As described in #900, there is no obvious way to create a nested serialization from a flat object. To fix this, fields.Nested should accept an optional from_parent argument. When true, the nested Schema is passed the parent object rather than the attribute at the field name. from_parent will default to False, which will maintain current behavior.

Any feedback on this idea? I plan on implementing it soon.

Thanks, James

[multimeric]

I also solved this using a (simpler) variant of @m-a-choquette’s method:

class SelfNested(fields.Nested):
    def get_value(self, obj, attr, accessor=None, default=missing):
        return obj

---

This let me use this schema:

class SampleFilterSchema(with_metaclass(SchemaMeta, BaseModelSchema)):
    class Meta:
        model = SampleFilter

    id = fields.Integer(attribute='sample_filter_id')
    type = fields.Constant('filter')
    attributes = SelfNested(Schema.from_dict(dict(
        tag=fields.String(attribute='sample_filter_tag'),
        name=fields.String(attribute='sample_filter_name'),
        public=fields.Boolean(attribute='is_public'),
        data=JsonString(attribute='sample_filter_data'),
        user=ResourceHyperlink(endpoint='rest_api.user', url_args=[
            'user_id',
        ])
    )))

(ignore my custom fields, they’re not important for this example)

Which transforms:

{  
   "user_id":1,
   "is_public":false,
   "sample_filter_name":"TestFilter",
   "sample_filter_data":"",
   "sample_filter_tag":"Global",
   "sample_filter_id":1
}

into

{
   "id":1,
   "attributes":{
      "user":"/rest_api/v1/users/1",
      "tag":"Global",
      "public":false,
      "name":"TestFilter",
      "data": ""
   }
}

aka, a valid response (or segment thereof) under the JSON API standard!

[m-a-choquette]

Hi, It feels a little weird to set a property on the object while serializing it.

To work around this, we did something like this:

def identity_accessor(attr, obj, default):
    return obj or default


class Section(fields.Nested):
    def get_value(self, attr, obj, accessor=None, default=missing):
        return super(Section, self).get_value(attr, obj,
                                              accessor=identity_accessor,
                                              default=default)

Each Section field receives the full object.