Question

假设我有两个模型，Group和Person，其中Group是Person的外键字段。在管理页面中，组被表示为Person管理员的下拉/选择字段。现在，我希望将选择的数量限制为，例如，五，并且应该根据他们的name对它们进行排序。

目前，我有以下代码：

class PersonAdmin(admin.ModelAdmin):
    form = PersonAdminForm
    ...
    def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
        if db_field.name == 'group':
            kwargs['queryset'] = Group.objects.all().order_by('name')[:5]

        return super(PersonAdmin, self).formfield_for_foreignkey(
            db_field, request, **kwargs)

class PersonAdminForm(forms.ModelForm):
    class Meta:
        model = Person

问题在于，当我尝试保存对象时，出现以下错误：AssertionError: Cannot filter a query once a slice has been taken.。

我已经搜索了这个错误，发现了很多SO线程，但是没有一个能帮我解决这个问题。

这是完整的堆栈跟踪：

Traceback (most recent call last):
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 583, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/utils/decorators.py", line 105, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/views/decorators/cache.py", line 52, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/contrib/admin/sites.py", line 206, in inner
    return view(request, *args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1453, in add_view
    return self.changeform_view(request, None, form_url, extra_context)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/utils/decorators.py", line 29, in _wrapper
    return bound_func(*args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/utils/decorators.py", line 105, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/utils/decorators.py", line 25, in bound_func
    return func.__get__(self, type(self))(*args2, **kwargs2)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/db/transaction.py", line 394, in inner
    return func(*args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1396, in changeform_view
    if form.is_valid():
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/forms/forms.py", line 162, in is_valid
    return self.is_bound and not bool(self.errors)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/forms/forms.py", line 154, in errors
    self.full_clean()
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/forms/forms.py", line 353, in full_clean
    self._clean_fields()
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/forms/forms.py", line 368, in _clean_fields
    value = field.clean(value)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/forms/fields.py", line 150, in clean
    value = self.to_python(value)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/forms/models.py", line 1185, in to_python
    value = self.queryset.get(**{key: value})
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/db/models/query.py", line 345, in get
    clone = self.filter(*args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/db/models/query.py", line 691, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/home/man/.virtualenvs/foo/local/lib/python2.7/site-packages/django/db/models/query.py", line 703, in _filter_or_exclude
    "Cannot filter a query once a slice has been taken."
AssertionError: Cannot filter a query once a slice has been taken.

Answer 1

您可以修改 init 方法中的查询集，找到前五个组ids，然后按pk __in进行过滤

class PersonAdminForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(PersonAdminForm, self).__init__(*args, **kwargs)
        group_ids = Group.objects.all().order_by('name').values_list('pk', flat=True)[:5]
        self.fields['group'].queryset = Group.objects.filter(pk__in=group_ids).order_by('name')

    group = forms.ModelChoiceField(queryset=None, empty_label=None)

    class Meta:
        model = Person
        fields = '__all__'

Answer 2

为避免过滤切片查询集的错误，您可能会获取前五个组，然后使用__in查找创建第二个查询集。这很hacky，但我认为它会起作用。

first_five = list(Group.objects.all().order_by('name')[:5].values('pk', flat=True))
queryset = Group.objects.filter(id__in=first_five).order_by('name')[:5]

如果您已有自定义模型表单，那么我认为此代码最干净的位置是表单的__init__方法，而不是覆盖formfield_for_foreignkey。

如何限制Django管理员的选择数量

2 个答案: