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]

Probably because I forgot to assign the task, my bad, lesson learned: do not sprint too much during a sprint !

This new PR fixes the bug inside ModelAdmin: ​https://github.com/django/django/pull/4822

[James Pic]

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

It's the same but rebased on master.

[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.