Support usage of 'source' in extra_kwargs.

[theosotr]

This commit fixes the issue when you set the keyword argument source
and your have not set the serializer fields explicitly. Then the
construction of field failed because there is not actually any model
field with that name.

However, you are still able to imply the name of model field by
taking advantage of the source keyword argument.

[xordoquy]

Hi, I think a test case here would help understand what this change brings to the table.

[theosotr]

Imagine the following scenario:

class MyModel(models.Model):
    model_field_name = models.CharField(max_length=255)

class MySerializer(serializers.ModelSerializer):
    class Meta(object):
        fields = ['api_field_name']
        extra_kwargs = {
            'api_field_name': {
                'source': 'model_field_name'
            }
        }
        model = models.MyModel

This code will break down with the following exception.

Traceback (most recent call last):
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/viewsets.py", line 87, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/views.py", line 474, in dispatch
    response = self.handle_exception(exc)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/views.py", line 434, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/views.py", line 471, in dispatch
    response = handler(request, *args, **kwargs)
  File "/home/thodoris/projects/apimas/apimas/modeling/adapters/drf/mixins.py", line 116, in list
    serializer = self.get_serializer(queryset, many=True)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/generics.py", line 111, in get_serializer
    return serializer_class(*args, **kwargs)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 102, in __new__
    return cls.many_init(*args, **kwargs)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 123, in many_init
    child_serializer = cls(*args, **kwargs)
  File "/home/thodoris/projects/apimas/apimas/modeling/adapters/drf/serializers.py", line 24, in __init__
    serializer_fields = self.fields
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 340, in fields
    for key, value in self.get_fields().items():
  File "/home/thodoris/projects/apimas/apimas/modeling/adapters/drf/serializers.py", line 100, in get_fields
    field_name, info, model, depth
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 1104, in build_field
    return self.build_unknown_field(field_name, model_class)
  File "/home/thodoris/projects/apimas/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 1211, in build_unknown_field
    (field_name, model_class.__name__)
ImproperlyConfigured: Field name `api_field_name` is not valid for model `MyModel`.

My commit fixes this issue so that I will no longer need to specify the api_field_name explicitly as follows:

class MySerializer(serializers.ModelSerializer):
    api_field_name = serializers.CharField(source='model_field_name')
    class Meta(object):
        fields = ['api_field_name']
        model = models.MyModel

[theosotr]

What about this?

[tomchristie]

Yup, seems perfectly reasonable.

[sccovey]

This doesn't seem to work for following related fields - I cannot do

extra_kwargs = {
            'name': {'source': 'related_field.name'},
            --or--
            'name': {'source': 'related_field__name'},
}

Does this seem correct? In my testing, an explicit field attribute was required

[xordoquy]

The ModelSerializer needs to generate the field from its model directly. It likely can not do so from related models.
Might be possible but I'm not sure the feature is worth the added complexity

[sshishov]

@xordoquy @tomchristie It would still be very reasonable and useful for a lot of cases.

For instance in many-to-many serialization, we provide the following data:

{
  "id": "<trhough_model_id>",
  "some_field": "<field_on_many_to_many>",
  "object_id": "<related_field_of_m2m>"  # <-- currently this serialization cannot be done using `extras`
}

In Python it will look like:

class M2MSerializer(ModelSerializer[M2MModel]):
    # object_id = CharField(source='related__external_id', help_text='...')  # <-- we are trying to avoid this one

    class Meta:
        extra_fields = {
            'id': {'source': 'external_id', 'help_text': '...'},
            'object_id': {'source': 'related__external_id', 'help_text': '...'},  # <-- this is not working and we have to create the object manually and we should keep it in sync also manually
            'some_field': {'source': 'another_field', 'help_text': '...'},
        }
        fields = ('id', 'object_id', 'some_field')

[auvipy]

@xordoquy @tomchristie It would still be very reasonable and useful for a lot of cases.

For instance in many-to-many serialization, we provide the following data:

{
  "id": "<trhough_model_id>",
  "some_field": "<field_on_many_to_many>",
  "object_id": "<related_field_of_m2m>"  # <-- currently this serialization cannot be done using `extras`
}

In Python it will look like:

class M2MSerializer(ModelSerializer[M2MModel]):
    # object_id = CharField(source='related__external_id', help_text='...')  # <-- we are trying to avoid this one

    class Meta:
        extra_fields = {
            'id': {'source': 'external_id', 'help_text': '...'},
            'object_id': {'source': 'related__external_id', 'help_text': '...'},  # <-- this is not working and we have to create the object manually and we should keep it in sync also manually
            'some_field': {'source': 'another_field', 'help_text': '...'},
        }
        fields = ('id', 'object_id', 'some_field')

you can come with a failing test case in a PR