CODEWITHSUNDEEP
pythondjango
Melvic Ybanez
Melvic Ybanez

Reputation: 2023

How to limit the number of choices in Django admin

Supposing I have two models, Group and Person, where Group is a foreign key field of Person. In the admin page, Group is represented as a dropdown/choice fields for the Person admin. Now, I want the number of choices to be limited to, say, five, and that they should be ordered according to their names.

Currently, I have the following code:

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

The problem is that when I try to save the object, I get the following error: AssertionError: Cannot filter a query once a slice has been taken..

I've searched for that error and found a lot of SO threads, but none of them seemed to help me with my case.

Here is the full stacktrace:

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.

Upvotes: 1

Views: 2076

Answers (2)

Satendra
Satendra

Reputation: 6865

You can modify queryset inside init method, find first five group ids and then filter on pk by __in lookup

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__'

Upvotes: 2

Alasdair
Alasdair

Reputation: 308779

To avoid the error about filtering a sliced queryset, you might have fetch the first five groups then create a second queryset with the __in lookup. It's hacky, but I think it will work.

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]

If you already have a custom model form, then I think the cleanest place for this code would be the form's __init__ method, instead of overriding formfield_for_foreignkey.

Upvotes: 1

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