Extract method for manual_fields processing

Allows reuse of logic to replace Field instances in a field list by `Field.name`.

Adds a utility function for the logic plus a wrapper method on `AutoSchema`.

Closes #5632

rest_framework/schemas/inspectors.py

@@ -105,6 +105,25 @@ def get_pk_description(model, model_field):
     )
 
 
+def update_fields(fields, update_with):
+    """
+    Update list of coreapi.Field instances, overwriting on `Field.name`.
+
+    Utility function to handle replacing coreapi.Field fields
+    from a list by name. Used to handle `manual_fields`.
+
+    Parameters:
+
+    * `fields`: list of `coreapi.Field` instances to update
+    * `update_with: list of `coreapi.Field` instances to add or replace.
+    """
+    by_name = {f.name: f for f in fields}
+    for f in update_with:
+        by_name[f.name] = f
+    fields = list(by_name.values())
+    return fields
+
+
 class ViewInspector(object):
     """
     Descriptor class on APIView.
@@ -181,11 +200,7 @@ class AutoSchema(ViewInspector):
         fields += self.get_pagination_fields(path, method)
         fields += self.get_filter_fields(path, method)
 
-        if self._manual_fields is not None:
+        fields = self.update_manual_fields(fields)
-            by_name = {f.name: f for f in fields}
-            for f in self._manual_fields:
-                by_name[f.name] = f
-            fields = list(by_name.values())
 
         if fields and any([field.location in ('form', 'body') for field in fields]):
             encoding = self.get_encoding(path, method)
@@ -379,6 +394,14 @@ class AutoSchema(ViewInspector):
             fields += filter_backend().get_schema_fields(self.view)
         return fields
 
+    def update_manual_fields(self, fields):
+        """
+        Adjust `fields` with `manual_fields`
+        """
+        if self._manual_fields is not None:
+            fields = update_fields(fields, self._manual_fields)
+        return fields
+
     def get_encoding(self, path, method):
         """
         Return the 'encoding' parameter to use for a given endpoint.