has_changed fails to compare model instance with primary key

[adsva]

I have a ModelForm with a ModelChoiceField that has a model instance as initial data. The form is rendered as an extra form in a formset and empty_permitted is True, but it still goes through validation since has_changed on the widget fails to equate the pk of the pre-selected field as POSTed with the model instance from the initial data. Maybe ModelChoiceField needs its own widget that handles this comparison?

[Russell Keith-Magee]

I'm not sure if a separate widget is required; all that is needed is to identify why an initial model is identified as a change in model data.

[tuxcanfly]

I've added a test case to reproduce this.

A workaround is to pass instance.pk instead of instance to initial.

I'm not sure how a widget can figure out that the initial data was a model instance so that
it can use the primary key while comparing.

I think this issue also affects ModelMultipleChoiceField.

[tuxcanfly]

Trying a fix by updating has_changed to check on both string and python values.

Added test for ModelMultipleChoiceField as well. Verified that all tests pass on sqlite.

Needs review: ​https://github.com/django/django/pull/660

[Claude Paroz]

I did some refactoring of the _has_changed code for #16612, and I included your tests which now also pass.

​https://github.com/django/django/pull/676

[Claude Paroz <claude@…>]

In 3318595c0bfeda9f6bae8aa11dda68647ae55fde:

Fixed #18898 -- Added tests with a fix for ModelMultipleChoiceField