title: Get the value of a read-only field in Serializer validation
body: <h3>Error ###<br /></h3>
  <p>TypeError at /api/answer/<br />
  '&gt;' not supported between instances of 'NoneType' and 'datetime.datetime'</p>
  <h3>serializer.py ###<br /></h3>
  <pre><code>class AnswerSerializer(serializers.ModelSerializer):
      class Meta:
           fields = ('exercise', 'answer_text', 'answer_file','datesend')
           read_only_fields = ('datesend',)
      
      def validate(self, attrs):
         date = attrs.get('datesend')
         ex = attrs['exercise']
         dateend = Exercise.objects.get(topic=ex.topic)
         if date &gt; dateend.exp_answer_date:
            raise serializers.ValidationError('Response time is over')
         return attrs
  </code></pre>