Django Foreign Key limit_choice_to with model object filter

Question

I'm trying to limit the choices for a foreign key in my django admin tool. I stumbled across the limit_choices_to option for the ForeignKey model: https://docs.djangoproject.com/en/3.2/ref/models/fields/#django.db.models.ForeignKey.limit_choices_to

In my models, I have a Question_logic that has an attribute ifLogic which is a foreign key of the model Options (which represent the possible answers of options of a question).

# Models.py

class Options(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    option = models.CharField(max_length=200)

class Question_logic(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
# error here:
    ifLogic = models.ForeignKey(Options, on_delete=models.CASCADE, limit_choices_to=Options.objects.filter(question=question))
    thenLogic = models.CharField(max_length=200)

However this approach does not work because apparently you can't reference an object in the model definition itself and I'm getting this error:

django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.

My logic is quite straightforward: I want the admin to chose only between the options that are relevant for the specific question. How do I implement this correctly?

Thanks.

Answer 1

Foreign key choices in the admin can be limited via ModelAdmin.formfield_for_foreignkey() . During form instantiation, the current object (ie QuestionLogic ) is stored on the request object that is available in the formfield_for_foreignkey() method.

Note that the object is not stored on the ModelAdmin object because that is not thread-safe.

class QuestionLogicAdmin(admin.ModelAdmin):
    def get_form(self, request, obj=None, **kwargs)
        # Store Question object on request for later retrieval.
        request._obj_ = obj 

        return super().get_form(request, question, **kwargs)
        
    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == "ifLogic" and request._obj_:
            question = request._obj_.question
            kwargs["queryset"] = Options.objects.filter(question=question)

PS: In Python, it is good practice to use CapitalizedWords naming convention for class names (ie QuestionLogic instead of Question_logic). Instance variables should be lowercase with words separated by underscores as necessary to improve readability (ie if_logic instead of ifLogic ). You can read up on that in the PEP8 styleguide .