title: Reverse relationships in Nested serializer in django rest framework
body: <p>My models.py is as as follows:</p>
  
  <pre><code>class Group(models.Model):
      name = models.CharField(max_length=50, unique=True)
  
  class Policy(models.Model):
      name =  models.CharField(max_length=50, unique=True)
      source_group =  models.ForeignKey(Group, related_name='source_group')
      destination_group = models.ForeignKey(Group, related_name='destination_group')
  </code></pre>
  
  <p>Since I have two foreign keys, pointing to the same model, I am using related name to avoid clashes.</p>
  
  <p>Now, when I try to create a serializer for Groups in order to list all Policies associated with it, I do the following:</p>
  
  <pre><code>class PolicySerializer(serializers.ModelSerializers):
      class Meta:
           model = Policy
           fields = "__all__"
  
  class GroupSerializer(serializers.ModelSerializer):
      policy = PolicySnippetSerializer(source ='source_group', many=True)
  
      class Meta:
          model = Group
          fields = ['id', 'name', 'policy']
  </code></pre>
  
  <p>However, this will give me only policies for a a source_group.How did i get all groups associated with a group, source and destination ? </p>