django - 在 Django QuerySet 中组合 prefetch_related() 和 only()

Question

我正在尝试执行以下查询:

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

好像不行。 Django 抛出以下异常:

'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'

如果我从 only() 方法中删除 childs__child_atrtibute 参数，一切都会按预期进行，但我的查询会返回不必要的数据。

Django 手册说我们可以将 only() 和 defer() 与来自 select_related() 的字段结合起来。

但是有没有一种方法可以将 only() 与来自 prefetch_related() 的字段一起使用？

我试图在我的代码后面使用 ChildModel.objects.only('child_attribute').all()，但它只会生成大量查询，而不是使用来自 的缓存结果prefetch_related().

我正在使用 Django 1.5。

Answer 1

我知道我可能来晚了，而不是根据问题的要求(Django 1.5)，但万一这对任何人有帮助......

从 Django 1.8 开始，使用 Prefetch对象，这可能可以实现如下:

ParentModel.objects.prefetch_related(
    Prefetch(
        'child_model',
        queryset=ChildModel.objects.all().only(
             'parent_attribute', 'childs__child_atrtibute'
        )
    )
)