How do I make a TabularInline required in Django admin?

Question

I want a TabularInline field in Django admin to be required. How do I proceed? Here's my code:

admin.py

class SchoolInline(TabularInline):
    model = School.labs.through
    extra = 1

class LaboratoryAdmin(ModelAdmin):
    inlines = [SchoolInline]

register(Lab, LaboratoryAdmin)

I simplified a lot the problem, but basically that's it. In result I get a drop-down list with all the schools. The problem is that this field isn't required, but I want it to be required. How can I do this the most simple way?

Answer 1

forms.py

# One form required
from django.core.exceptions import ValidationError
from django.forms.models import BaseInlineFormSet    

class AtLeastOneFormSet(BaseInlineFormSet):
    def clean(self):
        super(AtLeastOneFormSet, self).clean()
        non_empty_forms = 0
        for form in self:
            if form.cleaned_data:
                non_empty_forms += 1
        if non_empty_forms - len(self.deleted_forms) < 1:
            raise ValidationError("Please fill at least one form.")

forms.py

# First form not empty and can not be deleted
from django.forms.models import BaseInlineFormSet    

class RequiredInlineFormSet(BaseInlineFormSet):
    def _construct_form(self, i, **kwargs):
        form = super(RequiredInlineFormSet, self)._construct_form(i, **kwargs)
        if i < 1:
            form.empty_permitted = False
        return form

You also need to change the view and remove the delete button for the first form as shown here: https://docs.djangoproject.com/en/dev/topics/forms/formsets/#manually-rendered-can-delete-and-can-order

admin.py

from django.contrib.admin import TabularInline

class SchoolInline(TabularInline):
    model = School.labs.through
    extra = 1
    formset = RequiredInlineFormSet # or AtLeastOneFormSet