Refs #29227 -- Simplified NullBooleanField by making it subclass BooleanField.

django/contrib/admin/filters.py

@@ -251,10 +251,7 @@ class BooleanFieldListFilter(FieldListFilter):
             }
 
 
-FieldListFilter.register(
-    lambda f: isinstance(f, (models.BooleanField, models.NullBooleanField)),
-    BooleanFieldListFilter
-)
+FieldListFilter.register(lambda f: isinstance(f, models.BooleanField), BooleanFieldListFilter)
 
 
 class ChoicesFieldListFilter(FieldListFilter):

django/contrib/admin/utils.py

@@ -396,7 +396,7 @@ def display_for_field(value, field, empty_value_display):
         return dict(field.flatchoices).get(value, empty_value_display)
     # BooleanField needs special-case null-handling, so it comes before the
     # general null test.
-    elif isinstance(field, (models.BooleanField, models.NullBooleanField)):
+    elif isinstance(field, models.BooleanField):
         return _boolean_icon(value)
     elif value is None:
         return empty_value_display

django/db/models/fields/__init__.py

@@ -1956,10 +1956,10 @@ class GenericIPAddressField(Field):
         })
 
 
-class NullBooleanField(Field):
-    empty_strings_allowed = False
+class NullBooleanField(BooleanField):
     default_error_messages = {
         'invalid': _("'%(value)s' value must be either None, True or False."),
+        'invalid_nullable': _("'%(value)s' value must be either None, True or False."),
     }
     description = _("Boolean (Either True, False or None)")
 
@@ -1977,35 +1977,6 @@ class NullBooleanField(Field):
     def get_internal_type(self):
         return "NullBooleanField"
 
-    def to_python(self, value):
-        if value is None:
-            return None
-        if value in (True, False):
-            return bool(value)
-        if value in ('None',):
-            return None
-        if value in ('t', 'True', '1'):
-            return True
-        if value in ('f', 'False', '0'):
-            return False
-        raise exceptions.ValidationError(
-            self.error_messages['invalid'],
-            code='invalid',
-            params={'value': value},
-        )
-
-    def get_prep_value(self, value):
-        value = super().get_prep_value(value)
-        if value is None:
-            return None
-        return self.to_python(value)
-
-    def formfield(self, **kwargs):
-        return super().formfield(**{
-            'form_class': forms.NullBooleanField,
-            **kwargs,
-        })
-
 
 class PositiveIntegerRelDbTypeMixin: