site_tps/content/forms.py

from django import forms

from .models import Content, Category


class CreateContent(forms.ModelForm):
    class Meta:
        model = Content
        fields = [
            'name',
            'category',
            'file',
        ]

    def __init__(self, school, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.instance.school = school

        already_created = map(lambda x:x.category.pk, school.content_set.select_related('category'))
        self.fields['category'].queryset = Category.objects.exclude(pk__in=already_created)


class ContentEdit(forms.ModelForm):
    class Meta:
        model = Content
        fields = [
            'name',
            'file'
        ]