Model formsets apply unique checks to blank, null fields

[Chris Adams]

This is similar to #13811 but applies to fields declared with unique=True rather than unique_together: a model formset will incorrectly raise a ValidationError during validate_unique if more than one of the objects have a null value. I noticed this on a site using admin list_editable with a blank, null field but the problem appears to affect any model formset.

[Dan LaManna]

Re: Making ModelForm unique validation ignore blank fields where field.blank=True

I believe this is an issue with BaseModel validation, specifically _perform_unique_checks.

I attached a patch that works for me, would love feedback on it (one line change).

[Joeri Bekker]

I think @dlams is touching a different issue than the original ticket. This specific issue seems invalid to me because blank fields, for example an empty string on a CharField, saved in the database should trigger unique checks in contrast to NULL values which mean "unknown" or "can be anything". An empty string ("") actually means only one thing, namely an empty string and can be unique.

Please open a new ticket for this issue if you feel the above rationale is incorrect and still have problems with this.

[Joeri Bekker]

Test showing the current behaviour, elaborating on ticket starter's issue.

[Joeri Bekker]

The original ticket from @acdha is about a formset raising a ValidationError if a model field that has unique=True and null=True and more than 1 object exists that already has NULL in this field.

I started writing a test for this issue to validate the bug and realized that you actually never post None or NULL but an empty string, thus rendering the issue invalid because an empty string should be checked for uniqueness as per my previous comment.

Attaching a diff file that adds a test that fails showing the topic starter's issue in case the ticket is reopened and a "fix" is preferred.

[Joeri Bekker]

Small addition: Work in #4136 is related to the underlying issue.