ModelChoiceFilter isnull not working

[skikd636]

When using the ModelChoiceFilter with an lookup_expr='isnull' django throws a value error exception
Cannot use None as a query value

This is caused by the ChoiceFilter.filter function that uses "None" instead of "True" as the value.

Line 169 in the filters.py module:
qs = self.get_method(qs)(**{'%s__%s' % (self.field_name, self.lookup_expr): None})

The ModelChoiceFilter should have it's own filter function defined as follows:

def filter(self, qs, value):
   if value != self.null_value:
      return super().filter(qs, value)
    
   qs = self.get_method(qs)(**{'%s__%s' % (self.field_name, self.lookup_expr): True})
   return qs.distinct() if self.distinct else qs

[skikd636]

Looking at this more. The error does get thrown when lookup_expr='isnull' but I think I was missing the point. I think this is more of a documentation issue. I assumed when reading the documentation that I needed to set lookup_expr='isnull' when I wanted to add a Null lookup to a ModelChoiceField. I finally understand that is not the case. I just need to add a null_label='NULL' which add the null value to the ModelChoiceField.

That is not clear in the documentation. The filter works as expected (able to find empty fields) if I just set that value.