[1.9.x] Separated natural key serialization tests

Backport of d59d3caf3 from master.

tests/serializers/models.py

@@ -157,3 +157,24 @@ class Player(models.Model):
 
     def __str__(self):
         return '%s (%d) playing for %s' % (self.name, self.rank, self.team.to_string())
+
+
+# ******** Models for test_natural.py ***********
+
+class NaturalKeyAnchorManager(models.Manager):
+    def get_by_natural_key(self, data):
+        return self.get(data=data)
+
+
+class NaturalKeyAnchor(models.Model):
+    objects = NaturalKeyAnchorManager()
+
+    data = models.CharField(max_length=100, unique=True)
+    title = models.CharField(max_length=100, null=True)
+
+    def natural_key(self):
+        return (self.data,)
+
+
+class FKDataNaturalKey(models.Model):
+    data = models.ForeignKey(NaturalKeyAnchor, models.SET_NULL, null=True)