在Django管理页面中过滤ManyToManyField

Question

The problem

I have two many to many fields ( disciplines , subjects ) showing in Excerpt Model and I would like to show in admin page only the subjects inside disciplines that corresponde.

Example

DISCIPLINE | SUBJECT
portugues  | article
portugues  | verbs
portugues  | nons
math       | numbers
math       | equations

DISCIPLINE = math

DISCIPLINE | SUBJECT
math       | numbers
math       | equations

How I partially solved

For that I'm using the function formfield_for_manytomany inside admin, it works well for filtering the objects, but I couldn't figure out how to filter the subject objects based on the discipline choosed,

What I Tried

I tried use a function in model.py get_disciplines for return the corresponding id, for then filtering the subjects, but this approach doesn't seem to work inside my admin proxy model, because raise an Error global name 'get_disciplines' is not defined probably cuz I can access outside model.py. I tried also declare this method inside admin proxy, but also doesnt work.

my code

# model.py
class Discipline(models.Model):
    id = models.CharField(max_length=15, primary_key=True)
    name = models.CharField(max_length=100)

class Subject(models.Model):
    id = models.CharField(max_length=15, primary_key=True)
    name = models.CharField(max_length=100)
    disciplines = models.ManyToManyField(Discipline)

class Excerpt(models.Model):
    discipline = models.ManyToManyField(Discipline)
    subjects = models.ManyToManyField(Subject)

    # my idea
    def get_disciplines(self):
        e = self.discipline.first()
        return e.id

# admin.py
class ExcerptTaggerAdmin(ImageCroppingMixin, admin.ModelAdmin):
    filter_horizontal = ('subjects','discipline')

    def formfield_for_manytomany(self, db_field, request, **kwargs):
        if db_field.name == "subjects":
            kwargs["queryset"] = Subject.objects.filter(disciplines = get_discipline)
        return super(ExcerptTaggerAdmin, self).formfield_for_manytomany(db_field, request, **kwargs)

class ExcerptTagger(Excerpt):
    class Meta:
        proxy = True

I'm not a pro in django, so I think can be simple for someone that has more experience, I hope you can help me

Answer 1

I found a solution but its not so nice, but its working

def formfield_for_manytomany(self, db_field, request, **kwargs):
    if db_field.name == "subjects":
        self_pub_id = request.resolver_match.args[0]

        d = Excerpt.objects.get(id=self_pub_id).discipline.first()

        kwargs["queryset"] = Subject.objects.filter(disciplines=d)

    return super(ExcerptTaggerAdmin, self).formfield_for_manytomany(db_field, request, **kwargs)

If someone has a better solution, I would appreciate