The list_display for CharField doesn't work for None value among choices

[Petr Dlouhý]

I have a following model field:

      DIRECTIONS = [
          ('trip_to', "From"),
          ('trip_from', "To"),
          (None, "None"),
          ('', "-"),
      ]

      direction = models.CharField(
          verbose_name="direction",
          choices=DIRECTIONS,
          max_length=20,
          null=True, blank=True)

I want to filter out the items with direction set to Null in my admin list display. Both the choices (None and '') are displayed in the filter, but none of them works. The None choice generates URL without the direction__exact parameter. The '' choice generates direction__exact= parameter, which also does not work.
What work is, if I set the URL parameter manually to direction__isnull=True, but the link is not generated that way.

My suggestion is, that when the CharField has null=True and blank=True, then there is always Unknown choice in list_filter and if the None or '' is in choices, than the choice is named that way.

[Vincenzo Pandolfo]

I am working on a patch here: ​https://github.com/vincepandolfo/django/tree/ticket_26277

I did not send a pull request yet because I am not sure if this is the behaviour we want it to have.
My current implementation allows a None value to be in the choices of a ChoicesFieldListFilter and, in the case of a CharField, if the empty string is among the choices and null=True it ignores it since empty strings are converted to Null in the database.

[Tim Graham]

Where is the empty string to null conversation happening? I think it should be possible to store both values and hence both should appear in the admin filter choices.

[Vincenzo Pandolfo]

I am not sure where this is happening but in the (limited) testing I have done it appears that when both blank and null are set to True a record created picking the empty string choice will be displayed as having the null value choice.

I will try testing with different databases to see where the issue lies, since I didn't find anything related in the Django source code.

[Tim Graham]

Left a few comments for improvement on the ​PR.

[Tim Graham <timograham@…>]

In 0693193:

Fixed #26277 -- Added support for null values in ChoicesFieldListFilter.