In Django, how to populate a generic relation from a single form field?

Question

In our application, we have model A that has fields for a generic relation, because it could be related to an instance of either one of two models ( B or C ).

When populating a non-generic ForeignKey field, we can use the ModelChoiceField. Our problem is we cannot find a MultipleModelsChoiceField, whose choices would be populated from querysets on both B and C .

How would one go about solving this problem by reusing as much as possible existing Django code ?

Answer 1

You can do something like this:

class YourForm(forms.ModelForm):
    your_field = forms.ChoiceField()

    def __init__(self, *args, **kwargs):
        super(YourForm, self).__init__(*args, **kwargs)
        your_generic_relations_objects = list(FirtsModel.object.all()) + list(SecondModel.objects.all())  # and etc
        object_choices = []
        for obj in your_generic_relations_objects:
            type_id = ContentType.objects.get_for_model(obj.__class__).id
            obj_id = obj.id
            form_value = 'type:%s-id:%s' % (type_id, obj_id)
            display_text = str(obj)
            object_choices.append([form_value, display_text])
        self.fields['your_field'] = forms.ChoiceField(choices=object_choices)

    class Meta:
        model = YourModel
        fields = [
            'your_field'  # and others
        ]

    def save(self, *args, **kwargs):
        object_string = self.cleaned_data['your_field']
        matches = re.match("type:(\d+)-id:(\d+)", object_string).groups()
        content_type_id = matches[0]
        object_id = matches[1]
        content_type = ContentType.objects.get(id=content_type_id)
        self.instance.object_id = object_id
        self.instance.content_type = content_type
        return super(YourForm, self).save()