Preserve custom ChoiceField mappings for model choices

rest_framework/serializers.py

@@ -1301,9 +1301,10 @@ def build_standard_field(self, field_name, model_field):
             field_kwargs['queryset'] = model_field.related_model.objects
 
         if 'choices' in field_kwargs:
-            # Fields with choices get coerced into `ChoiceField`
-            # instead of using their regular typed field.
-            field_class = self.serializer_choice_field
+            # Fields with choices default to `ChoiceField`, unless a custom
+            # `ChoiceField` subclass is already mapped for the model field.
+            if not issubclass(field_class, self.serializer_choice_field):
+                field_class = self.serializer_choice_field
             # Some model fields may introduce kwargs that would not be valid
             # for the choice field. We need to strip these out.
             # Eg. models.DecimalField(max_digits=3, decimal_places=1, choices=DECIMAL_CHOICES)

tests/test_model_serializer.py

@@ -369,6 +369,34 @@ class Meta:
         assert choices_field_with_nonstandard_args.choices
         assert choices_field_with_nonstandard_args.read_only is False
 
+    def test_custom_choice_field_mapping(self):
+        class CustomModelField(models.CharField):
+            pass
+
+        class CustomChoiceField(serializers.ChoiceField):
+            pass
+
+        class CustomChoiceModel(models.Model):
+            field = CustomModelField(
+                max_length=10,
+                choices=COLOR_CHOICES,
+            )
+
+        class TestSerializer(serializers.ModelSerializer):
+            serializer_field_mapping = {
+                **serializers.ModelSerializer.serializer_field_mapping,
+                CustomModelField: CustomChoiceField,
+            }
+
+            class Meta:
+                model = CustomChoiceModel
+                fields = ('field',)
+
+        field = TestSerializer().fields['field']
+
+        assert isinstance(field, CustomChoiceField)
+        assert list(field.choices) == [choice[0] for choice in COLOR_CHOICES]
+
     def test_non_editable_choice_field(self):
         class ExampleSerializer(serializers.ModelSerializer):
             class Meta: