I try to create an object via a POST request. This works very well, now to my problem:

I want the response to have the field expanded by default (in the direct response to the post request).
When trying this either via http://localhost:8000/api/cities?expand=country or setting extensions_expand = {'country'} on the view (my preferred way), i get a validation error "country": ["This field is required." ].

So using extensions_expand={'country'} results in an error on POST (and also PUT i think). I can bypass this error by using get_extensions_mixin_context on the view

 def get_extensions_mixin_context(self):
 context = super(CityViewSet, self).get_extensions_mixin_context()
 if self.request.method in ['POST', 'PUT']:
 context['expand'] = set()
 return context

This works at least, but still doesn't produce the desired result of having country expanded in the post response.

I also have a repo ready for reproduction: https://github.com/BerniWittmann/drf_serializer_ext
Just try to post something to localhost:8000/api/cities. The desired response would be:

{
 "url": "http://localhost:8000/api/cities/1/",
 "name": "Berlin",
 "id": 1,
 "country": {
 "url": "http://localhost:8000/api/countries/1/",
 "name": "Germany",
 "id": 1,
 "code": "DE"
 },
 "country_id": 1
}

If anything is unclear, let me know :)