"在序列化器验证中获取只读字段的值"

Question

错误 ＃＃＃

/api/answer/ 处的类型错误
'NoneType' 和 'datetime.datetime' 的实例之间不支持 '>'

序列化程序.py ###

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 > dateend.exp_answer_date:
          raise serializers.ValidationError('Response time is over')
       return attrs