title: Is there a way to maintain the serializer order?
body: <p>If I write this order for <code>AddressRegionDetailSerializer</code> and <code>AvailableAreaSerializer</code>:</p>
  
  <pre><code>class AddressRegionDetailSerializer(ModelSerializer):
      """
      地域详情（包含可用区域）
      """
      availableareas = AvailableAreaSerializer(many=True, read_only=True)
      class Meta:
          model = AddressRegion
          fields = "__all__"
  
  class AvailableAreaSerializer(ModelSerializer):
      """
      可用地区
      """
  
      class Meta:
          model = AvailableArea
          fields = "__all__"
  </code></pre>
  
  <p>There will report NameError issue:</p>
  
  <pre><code>NameError: name 'AvailableAreaSerializer' is not defined
  </code></pre>
  
  <p>in this line:</p>
  
  <pre><code>availableareas = AvailableAreaSerializer(many=True, read_only=True)    
  </code></pre>
  
  <p>So, I must put the <code>AvailableAreaSerializer</code> in the front. 
  But however in my idea, I want to write the Serializer as the Models order, I don't want to break this law. </p>
  
  <p>So, is there a simple way to maintain this order?  </p>