title: Foreign key value in Django REST Framework
body: <p><strong>models.py:</strong></p>
  
  <pre><code>class Station(models.Model):
      station = models.CharField()
  
  class Flat(models.Model):
      station = models.ForeignKey(Station, related_name="metro")
      # another fields
  </code></pre>
  
  <p>Then in <strong>serializers.py</strong>:</p>
  
  <pre><code>class StationSerializer(serializers.ModelSerializer):
      station = serializers.RelatedField(read_only=True)
  
      class Meta:
          model = Station
  
  
  class FlatSerializer(serializers.ModelSerializer):
      station_name = serializers.RelatedField(source='station', read_only=True)
  
      class Meta:
          model = Flat
          fields = ('station_name',)
  </code></pre>
  
  <p>And I have an error:</p>
  
  <blockquote>
    <p>NotImplementedError: <code>RelatedField.to_representation()</code> must be implemented.
    If you are upgrading from REST framework version 2 you might want <code>ReadOnlyField</code>.<br>
    I read <a href="https://stackoverflow.com/questions/17280007/retreiving-a-foreign-key-value-with-django-rest-framework-serializers">this</a>, but it does not help me.<br>
    How to fix that?<br>
    Thanks!</p>
  </blockquote>