title: Django-REST-Framework - create element with list of elements
body: <p>I have a 2 models. <code>A</code> and <code>B</code>. model <code>B</code> has a ForeignKey field to model <code>A</code> so in practice, <code>A</code> has a list of <code>B</code> elements.<br/>
  I'm posting the following data to a Django-REST-Framework api.</p>
  
  <pre><code>{"a_field_1": "value1", "bs": [{"b_field_1": "value1", "b_field_2": "value2"}]}
  </code></pre>
  
  <p>These are my Serializers:</p>
  
  <pre><code>class BSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = B
          fields = ('b_field_1', 'b_field_2')
  
  
  class ASerializer(serializers.ModelSerializer):
  
      bs = BSerializer(many=True)
  
      class Meta:
          model = A
          fields = ('a_field_1', 'bs')
  </code></pre>
  
  <p>When I post to my API i get this error:</p>
  
  <pre><code>The `.create()` method does not support writable nestedfields by default.
  Write an explicit `.create()` method for serializer `app.serializers.ASerializer`, or set `read_only=True` on nested serializer fields.
  </code></pre>
  
  <p>I found <code>drf_compound_fields</code> package and wrapped the <code>bs</code> field in the ASerializer like so</p>
  
  <pre><code>bs = ListField(BSerializer(many=True))
  </code></pre>
  
  <p>This created the A object but didn't create the composite B elements.
  When i changed the field name to <code>b_set</code> (instead of <code>bs</code>) i got the following error:</p>
  
  <pre><code>TypeError: 'RelatedManager' object is not iterable
  </code></pre>
  
  <p>What is the proper way to serialize and create a single element with a list of composite elements. </p>