diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 40d768975..1fea13802 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -410,10 +410,12 @@ class ModelSerializer(Serializer):
         info = model_meta.get_field_info(self.opts.model)
         ret = SortedDict()
 
+        # URL field
         serializer_url_field = self.get_url_field()
         if serializer_url_field:
             ret[api_settings.URL_FIELD_NAME] = serializer_url_field
 
+        # Primary key field
         field_name = info.pk.name
         serializer_pk_field = self.get_pk_field(field_name, info.pk)
         if serializer_pk_field:
diff --git a/tests/test_model_field_mappings.py b/tests/test_model_field_mappings.py
index 957bf6613..bae63e5a7 100644
--- a/tests/test_model_field_mappings.py
+++ b/tests/test_model_field_mappings.py
@@ -14,7 +14,7 @@ def dedent(blocktext):
     return '\n'.join([line[12:] for line in blocktext.splitlines()[1:-1]])
 
 
-# Models for testing regular field mapping
+# Testing regular field mappings
 
 class RegularFieldsModel(models.Model):
     auto_field = models.AutoField(primary_key=True)
@@ -70,7 +70,7 @@ class TestRegularFieldMappings(TestCase):
         self.assertEqual(repr(TestSerializer()), expected)
 
 
-# Model for testing relational field mapping
+# Testing relational field mappings
 
 class ForeignKeyTargetModel(models.Model):
     name = models.CharField(max_length=100)