Overridden field in modelform used in modeladmin which name is returned by get_readonly_fields

[James Pic]

A Model ForeignKey field will appear twice in the admin if the following conditions are met:

• its ModelAdmin uses a custom ModelForm,
• the ModelForm overrides that field,
• its ModelAdmin.get_readonly_fields returns that field in edit mode.

Example:
​https://github.com/jpic/double_readonly_field/raw/master/demo.png

Test project reproducing this issue:

• ​https://github.com/jpic/double_readonly_field

Related:

• ​https://github.com/yourlabs/django-autocomplete-light/issues/235
• ​https://github.com/yourlabs/django-autocomplete-light/issues/440

[James Pic]

This is fixed by : ​https://github.com/django/django/pull/4747/

Ready for review and probably for merge, since it's really trivial.

[David Gouldin]

Looks like this is happening because ModelFormMetaclass.__new__ doesn't expect the form class's declared fields to also be in its exclude opts. Normally, that would be a reasonable assumption. (Why would you exclude a field you've bothered to explicitly deeclared?) But in this case, it's the admin which is excluding the field, so the assumption breaks. Patch coming.

[David Gouldin]

Oops, just realized there's already a PR for this. Ignore me.

[James Pic]

I got a bit confused by another problem and the PR above has become unrelated.

The new PR which fixes the bug in ModelAdmin is: ​https://github.com/django/django/pull/4822 (I wish I had contributed something more elegant lol)

[James Pic]

Closed PR 4822 in favor of 4824: ​https://github.com/django/django/pull/4824

It's the same but rebased on master. It seems like the right implementation indeed, which is "encapsulated in the class that provides the readonly feature" (and with all the awesomeness that's in type())

[Tim Graham <timograham@…>]

In fedef7b2:

Fixed #24908 -- Fixed duplicate readonly field rendering.

ModelAdmin added readonly_fields to exclude, but would not undeclare
them if they were overridden.