Django-restframework: How to clear/empty a ManyToMany relation with FormData

djangonaut

I'm editing Django user instances via django-rest-framework, it works well, including setting/changing group membership. But I can't manage to clear/empty the user from all groups.

This works for me on the shell with httpie:

http -j PATCH :8000/api/accounts/user/1/  groups:='[]'

This is what I try within the browser:

var data = new FormData(); 
data.append('groups', JSON.stringify([])); // not working

$.ajax({
  dataType: 'json',
  url: '/api/accounts/user/1/',
  type: 'PATCH',
  processData: false, 
  contentType: false,
  data: data
});

The form data looks like:

------WebKitFormBoundaryJu7QPC8AcBTpWEKJ
Content-Disposition: form-data; name="groups"

[]

The response is:

{"groups": ["Incorrect type.  Expected pk value, received unicode."]}

djangonaut

I guess the problem is that FormData cannot transfer raw JSON like httpie. restframework treats them differently:

def field_from_native(self, data, files, field_name, into):
    ...
    if self.many:
        try:
            # Form data
            value = data.getlist(field_name)
            if value == [''] or value == []:
                raise KeyError
        except AttributeError:
            # Non-form data
            value = data[field_name]

This custom method makes it work but I still don't like the approach. To clear I append null to the FormData:

data.append('groups', null);

And make it an empty list on the backend:

class FormDataPrimaryKeyRelatedField(serializers.PrimaryKeyRelatedField):
    """
     Custom PrimaryKeyRelatedField which allows clearing via form-data using
     JS's 'null' as to clearing value.
    """
    def field_from_native(self, data, files, field_name, into):
        data = dict(data)
        if self.many:
            if data.get(field_name) == [u'null']:
                data[field_name] = []

        super(FormDataPrimaryKeyRelatedField, self).field_from_native(data, files, field_name, into)


class UserSerializer(serializers.ModelSerializer):

    groups = FormDataPrimaryKeyRelatedField(many=True, source='groups', required=False)

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