django admin make a field read-only when modifying obj but required when adding new obj

Question

In admin I would like to disable a field when modifying object, but make it required when adding new object.

Whats the django way to go about this one?

Answer 1

You can do this by overriding the formfield_for_foreignkey method of the ModelAdmin:

from django import forms
from django.contrib import admin

from yourproject.yourapp.models import YourModel

class YourModelAdmin(admin.ModelAdmin):

    class Meta:
        model = YourModel

    def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
        # Name of your field here
        if db_field.name == 'add_only':
            if request:
                add_opts = (self._meta.app_label, self._meta.module_name)
                add = u'/admin/%s/%s/add/' % add_opts
                if request.META['PATH_INFO'] == add:
                    field = db_field.formfield(**kwargs)
                else:
                    kwargs['widget'] = forms.HiddenInput()
                    field = db_field.formfield(**kwargs)
            return field
        return admin.ModelAdmin(self, db_field, request, **kwargs)