Combinaison de prefetch_related() et only() dans un Django QuerySet - Stack Overflow

I am trying to execute the following query:

ParentModel.objects.prefetch_related('child_model').only('parent_attribute', 'childs__child_atrtibute').all()

It doesn't seem to work. Django throws the following exception:

'RelatedObject' object has no attribute 'rel'

Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/Library/Python/2.7/site-packages/django/contrib/auth/decorators.py", line 25, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/Library/Python/2.7/site-packages/django/contrib/auth/decorators.py", line 25, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/Library/Python/2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/…path_to_my_app…/models.py", line 123, in _nested_commit_on_success
    return commit_on_success(*args, **kwds)
  File "/Library/Python/2.7/site-packages/django/db/transaction.py", line 223, in inner
    return func(*args, **kwargs)
  File "/…path_to_my_app…/views.py", line 1047, in my_veiw_function
    MyParentModel.objects.prefetch_related('my_child_model').only('my_parent_attribute', 'my_child_model__my_child_attribute').all():
  File "/Library/Python/2.7/site-packages/django/db/models/query.py", line 102, in __iter__
    len(self)
  File "/Library/Python/2.7/site-packages/django/db/models/query.py", line 90, in __len__
    self._result_cache = list(self.iterator())
  File "/Library/Python/2.7/site-packages/django/db/models/query.py", line 258, in iterator
    only_load = self.query.get_loaded_field_names()
  File "/Library/Python/2.7/site-packages/django/db/models/sql/query.py", line 1888, in get_loaded_field_names
    self.deferred_to_data(collection, self.get_loaded_field_names_cb)
  File "/Library/Python/2.7/site-packages/django/db/models/sql/query.py", line 591, in deferred_to_data
    cur_model = source.rel.to
AttributeError: 'RelatedObject' object has no attribute 'rel'

If I remove the childs__child_atrtibute argument from the only() method, everything works as expected, but my query returns unnecessary data.

Django manual says we can combine only() and defer() with fields from select_related().

But is there a way to use only() with fields from prefetch_related()?

I tried to rather use ChildModel.objects.only('child_attribute').all() later in my code, but it just generates tons of queries instead of using cached results from prefetch_related().

I am using Django 1.5.

I am trying to execute the following query:

ParentModel.objects.prefetch_related('child_model').only('parent_attribute', 'childs__child_atrtibute').all()

It doesn't seem to work. Django throws the following exception:

'RelatedObject' object has no attribute 'rel'

Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/Library/Python/2.7/site-packages/django/contrib/auth/decorators.py", line 25, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/Library/Python/2.7/site-packages/django/contrib/auth/decorators.py", line 25, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/Library/Python/2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/…path_to_my_app…/models.py", line 123, in _nested_commit_on_success
    return commit_on_success(*args, **kwds)
  File "/Library/Python/2.7/site-packages/django/db/transaction.py", line 223, in inner
    return func(*args, **kwargs)
  File "/…path_to_my_app…/views.py", line 1047, in my_veiw_function
    MyParentModel.objects.prefetch_related('my_child_model').only('my_parent_attribute', 'my_child_model__my_child_attribute').all():
  File "/Library/Python/2.7/site-packages/django/db/models/query.py", line 102, in __iter__
    len(self)
  File "/Library/Python/2.7/site-packages/django/db/models/query.py", line 90, in __len__
    self._result_cache = list(self.iterator())
  File "/Library/Python/2.7/site-packages/django/db/models/query.py", line 258, in iterator
    only_load = self.query.get_loaded_field_names()
  File "/Library/Python/2.7/site-packages/django/db/models/sql/query.py", line 1888, in get_loaded_field_names
    self.deferred_to_data(collection, self.get_loaded_field_names_cb)
  File "/Library/Python/2.7/site-packages/django/db/models/sql/query.py", line 591, in deferred_to_data
    cur_model = source.rel.to
AttributeError: 'RelatedObject' object has no attribute 'rel'

If I remove the childs__child_atrtibute argument from the only() method, everything works as expected, but my query returns unnecessary data.

Django manual says we can combine only() and defer() with fields from select_related().

But is there a way to use only() with fields from prefetch_related()?

I tried to rather use ChildModel.objects.only('child_attribute').all() later in my code, but it just generates tons of queries instead of using cached results from prefetch_related().

I am using Django 1.5.

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