request additional fields from related object in django queryset

Question

say I've got two models, Parent and Child.

Child model has a fk relation to Parent

Class Child(models.Model):
     parent = models.ForeignKey('Parent', related_name='family')

I want to write a queryset for Child and serialize it to geojson, however, I'd also like the queryset to include some additional fields from Parent beyond id. perhaps I'm not clear on select_related().

This offers functionally the same query with no additional fields from Parent:

qs = Child.objects.filter(...).select_related('parent')

props = {
    'geoJson' : serializers.serialize('geojson', list(qs)),
}

if I make it .select_related('parent').values(...) for the fields I want, the view errs on req:

'dict' object has no attribute '_meta'

.select_related('parent').only(...) throws:

Field Child.parent cannot be both deferred and traversed using select_related at the same time

what's the right way to structure that queryset to request extra fields from the related object? what am I misunderstanding about selected_related in queries? thanks

.values() Traceback:

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
  41.             response = get_response(request)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/core/handlers/base.py" in _legacy_get_response
  249.             response = self._get_response(request)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/channels/handler.py" in process_exception_by_middleware
  237.             return super(AsgiHandler, self).process_exception_by_middleware(exception, request)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File ".pyenv/versions/3.6.3/lib/python3.6/contextlib.py" in inner
  52.                 return func(*args, **kwds)

File "project/utils/helpers.py" in _decorated
  29.             return view(request, *args, **kwargs)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/htmlmin/decorators.py" in minify
  13.         response = f(*args, **kwargs)

File "/causes/views.py" in cause
  197.         'activityJson' : serializers.serialize('geojson', list(qs)),

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/core/serializers/__init__.py" in serialize
  129.     s.serialize(queryset, **options)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/core/serializers/base.py" in serialize
  81.             self.start_object(obj)

File ".pyenv/versions/3.6.3/lib/python3.6/site-packages/django/contrib/gis/serializers/geojson.py" in start_object
  35.             for field in obj._meta.fields:

Daniel Roseman · Accepted Answer · 2019-02-28 12:55:10Z

1

Your misunderstanding is that this doesn't have anything whatsoever to do with select_related. That method is for making more efficient queries only, and would not in any way affect what data is returned from a serializer.

Django's built-in serializers are very basic, and don't support nested objects like this. If that's what you want, you should use Django REST Framework, which implements much more fully-featured serializers which can serialize nested objects in a variety of ways.

answered Feb 28, 2019 at 12:55

Daniel Roseman

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