Question

Let’s assume that I have following models:

class ScoutBook(models.Model):
       troop = models.ForeignKey('Dictionary', limit_choices_to={'type' : 'Troop'},  related_name='+', blank=True, null=True)

class Dictionary(models.Model): 
     name = models.CharField(max_length=CHAR_FIELD_MAX_LEN, verbose_name="Nazwa")
     active = models.BooleanField(verbose_name="Aktywny")
     type = models.CharField(max_length=CHAR_FIELD_MAX_LEN, choices=DICTIONARY_CHOICES)

and I want to implement following logic:

when creating ScoutBook allow users to select only active troops, and when editing allow to select active troops or allow user to leave value unchanged (even if the troop is inactive). If I use limit_choices_to = {..., 'active' = True} troop that is inactive is absent from combo box in django admin.

So to be clear: let’s assume that there are four troops in this system: Troop1, Troop2 and InactiveTroop, InactiveTroop2. On model creation I would like user to be able to choose Troop1 and Troop2. If model has troop field set to InactiveTroop2, I would like user to be able to choose between InactiveTroop2, Troop1 and Troop2.

I was looking at the django forms api and I didn’t found obvious way do this. Moreover, in the application I’m developing there will be many such fields and many such models — so solution must be pain free. I would rather not create new Form class for every model. I will be using mostly django admin to enable editing the database, and some read only views that will just list entries.

Ideally I would like to encapsulate this functionality in some custom field — but fields have access to model instance on validate and save phase — so I dont have access to it when I produce formfield.

Answer 1

Well I had to hook into ModelForm creation. Attached Form inspects it’s fields and if specific conditions are met it replaces model field queryset.

    class DictionayModelForm(ModelForm):
        def __init__(self, *largs, **kwargs):
            super(DictionayModelForm, self).__init__(*largs, **kwargs)
            if self.instance and self.instance.pk is not None:
                for f in self.instance._meta.fields:
                    if isinstance(f, models.ForeignKey) and issubclass(f.rel.to, Dictionary):
                        model_field = self.fields[f.name]
                        value = getattr(self.instance, f.name, None)
                        if value and value not in model_field.choices:
                            model_field.queryset = Dictionary.objects.filter(Q(**f.rel.limit_choices_to) | Q(id = value.id))