build_field, build_final_kwargs

rest_framework/serializers.py

@@ -908,7 +908,22 @@ class ModelSerializer(Serializer):
                 ret[field_name] = declared_fields[field_name]
                 continue
 
-            elif field_name in info.fields_and_pk:
+            # Determine the serializer field class and keyword arguments.
+            field_cls, kwargs = self.build_field(field_name, info, model, depth)
+
+            # Populate any kwargs defined in `Meta.extra_kwargs`
+            kwargs = self.build_final_kwargs(kwargs, extra_kwargs, field_name)
+
+            # Create the serializer field.
+            ret[field_name] = field_cls(**kwargs)
+
+        # Add in any hidden fields.
+        ret.update(hidden_fields)
+
+        return ret
+
+    def build_field(self, field_name, info, model, depth):
+        if field_name in info.fields_and_pk:
             # Create regular model fields.
             model_field = info.fields_and_pk[field_name]
             field_cls = self._field_mapping[model_field]
@@ -955,8 +970,15 @@ class ModelSerializer(Serializer):
                 (field_name, model.__class__.__name__)
             )
 
-            # Populate any kwargs defined in `Meta.extra_kwargs`
+        return field_cls, kwargs
+
+    def build_final_kwargs(self, kwargs, extra_kwargs, field_name):
+        """
+        Include an 'extra_kwargs' that have been included for this field,
+        possibly removing any incompatible existing keyword arguments.
+        """
         extras = extra_kwargs.get(field_name, {})
+
         if extras.get('read_only', False):
             for attr in [
                 'required', 'default', 'allow_blank', 'allow_null',
@@ -970,12 +992,7 @@ class ModelSerializer(Serializer):
 
         kwargs.update(extras)
 
-            # Create the serializer field.
-            ret[field_name] = field_cls(**kwargs)
-
-        ret.update(hidden_fields)
-
-        return ret
+        return kwargs
 
     def _get_model_fields(self, field_names, declared_fields, extra_kwargs):
         """