Fix inability to override get_FIELD_display methods

Ensure custom get_FIELD_display() overrides are respected for model fields with choices.
This restores behavior present before Django 2.2.

Fixes # (referenced issue in description).

django/db/models/fields/__init__.py

@@ -763,8 +763,10 @@ class Field(RegisterLookupMixin):
             if not getattr(cls, self.attname, None):
                 setattr(cls, self.attname, self.descriptor_class(self))
         if self.choices is not None:
-            setattr(cls, 'get_%s_display' % self.name,
-                    partialmethod(cls._get_FIELD_display, field=self))
+            # Check that a get_FIELD_display method hasn't been manually defined.
+            if not hasattr(cls, 'get_%s_display' % self.name):
+                setattr(cls, 'get_%s_display' % self.name,
+                        partialmethod(cls._get_FIELD_display, field=self))
 
     def get_filter_kwargs_for_object(self, obj):
         """

tests/model_fields/models.py

@@ -405,3 +405,11 @@ class UUIDChild(PrimaryKeyUUIDModel):
 
 class UUIDGrandchild(UUIDChild):
     pass
+
+
+class CustomGetDisplayModel(models.Model):
+    """Test model for overriding get_FIELD_display()."""
+    foo_bar = models.CharField(max_length=10, choices=[(1, 'foo'), (2, 'bar')])
+
+    def get_foo_bar_display(self):
+        return "something"

tests/model_fields/tests.py

@@ -7,8 +7,8 @@ from django.test import SimpleTestCase, TestCase
 from django.utils.functional import lazy
 
 from .models import (
-    Bar, Choiceful, Foo, RenamedField, VerboseNameField, Whiz, WhizDelayed,
-    WhizIter, WhizIterEmpty,
+    Bar, Choiceful, CustomGetDisplayModel, Foo, RenamedField, VerboseNameField,
+    Whiz, WhizDelayed, WhizIter, WhizIterEmpty,
 )
 
 
@@ -186,6 +186,16 @@ class GetFieldDisplayTests(SimpleTestCase):
         self.assertIsNone(WhizIterEmpty(c=None).c)         # Blank value
         self.assertEqual(WhizIterEmpty(c='').c, '')        # Empty value
 
+    def test_overriding_get_FIELD_display(self):
+        """
+        Can override the get_FIELD_display() method.
+        """
+        obj = CustomGetDisplayModel(foo_bar=1)
+        # Should use the overridden method which always returns "something"
+        self.assertEqual(obj.get_foo_bar_display(), 'something')
+        obj.foo_bar = 2
+        self.assertEqual(obj.get_foo_bar_display(), 'something')
+
 
 class GetChoicesTests(SimpleTestCase):