add required argument in get_attribute

rest_framework/fields.py

@@ -48,7 +48,7 @@ def is_simple_callable(obj):
     return len_args <= len_defaults
 
 
-def get_attribute(instance, attrs):
+def get_attribute(instance, attrs, required):
     """
     Similar to Python's built in `getattr(instance, attr)`,
     but takes a list of nested attributes, instead of a single attribute.
@@ -69,6 +69,8 @@ def get_attribute(instance, attrs):
         except ObjectDoesNotExist:
             return None
         except AttributeError as exc:
+            if not required:
+                return None
             raise exc
         if is_simple_callable(instance):
             instance = instance()
@@ -277,7 +279,7 @@ class Field(object):
         Given the *outgoing* object instance, return the primitive value
         that should be used for this field.
         """
-        return get_attribute(instance, self.source_attrs)
+        return get_attribute(instance, self.source_attrs, self.required)
 
     def get_default(self):
         """