title: Get foreign serialized ReadOnlyField on Django Rest Framework
body: <p>Currently I have a serializer file like so:</p>
  <pre><code>class BotSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = Bot
          fields = ['id', 'bot_name', 'comment', 'bot_activities']
          depth = 1
  
  
  class BotActivitySerializer(serializers.ModelSerializer):
      bot_name = serializers.ReadOnlyField(source='bot.bot_name', read_only=True)
      started_date_activity = serializers.DateTimeField(source='started_at', format=&quot;%m/%d/%Y %H:%M:%S&quot;, required=False, read_only=True)
      finished_date_activity = serializers.DateTimeField(source='finished_at', format=&quot;%m/%d/%Y %H:%M:%S&quot;, required=False, read_only=True)
  
      class Meta:
          model = BotActivity
          fields = ['started_at', 'started_date_activity', 'finished_at', 'finished_date_activity', 'bot', 'bot_name', 'bot_activity_status']
  </code></pre>
  <p>When I call the Bot seriliazer I get:</p>
  <pre><code>{
      &quot;id&quot;: 1,
      &quot;bot_name&quot;: &quot;HTTP header checker&quot;,
      &quot;comment&quot;: &quot;testtest&quot;,
      &quot;bot_activities&quot;: [
          {
              &quot;id&quot;: 1,
              &quot;started_at&quot;: &quot;2020-11-04T15:47:26.168212Z&quot;,
              &quot;finished_at&quot;: null,
              &quot;bot_activity_status&quot;: &quot;In Progress&quot;,
              &quot;bot&quot;: 1
          }
      ]
  }
  </code></pre>
  <p>But I need to also get the started_date_activity and finished_date_activity fields.
  What to do to get their value too?
  Thank you</p>