Add support for serializing models with m2m related fields

- In both ManyRelatedField, provide an empty return when trying to
  access a relation field if the instance in question has no PK (so
  likely hasn't been inserted yet)
- Add relevant tests
- Without these changes, exceptions would be raised when trying to
  serialize the uncreated models as it is impossible to query
  relations without a PK
- Add test to ensure RelatedField does not regress as currently 
  supports being serialized with and unsaved model

1 files changed, 4 insertions, 0 deletions

diff --git a/rest_framework/relations.py b/rest_framework/relations.py
index 0b7c9d86..3a966c5b 100644
--- a/rest_framework/relations.py
+++ b/rest_framework/relations.py
@@ -360,6 +360,10 @@ class ManyRelatedField(Field):
         ]
 
     def get_attribute(self, instance):
+        # Can't have any relationships if not created
+        if not instance.pk:
+            return []
+
         relationship = get_attribute(instance, self.source_attrs)
         return relationship.all() if (hasattr(relationship, 'all')) else relationship