Django rest框架，PUT和Patch创建新对象而不是对其进行更新

Question

我正在尝试使用外键在模型上进行Patch / Put请求。 我覆盖了create和update方法以更新实例：

class CowManagementSerializer(serializers.ModelSerializer):
    group = serializers.CharField(required=True, allow_null=False)

    class Meta:
        model = Cow
        fields = ['management_id', 'eid_number', 'group']

    def validate(self, attrs):
        return attrs

    def create(self, validated_data):
        try:
            group_id = validated_data['group']
            eid_number = validated_data['eid_number']
            management_id = validated_data['management_id']
            print(group_id)
            group = Group.objects.get(group_id=group_id, farm__user=self.context['request'].user)
            Cow.objects.create(group=group, eid_number=eid_number, management_id=management_id)
            return validated_data
        except Group.DoesNotExist:
            raise serializers.ValidationError("Group with this name does not exist!")

    def update(self, instance, validated_data):
        group_id = validated_data.pop('group')
        group = Group.objects.get(group_id=group_id, farm__user=self.context['request'].user)
        instance.group = group
        instance.management_id = validated_data['management_id']
        instance.eid_number = validated_data['eid_number']
        instance.save()
        return instance

制作补丁或发出请求只会创建另一个具有相同数据的对象。

发送相同的请求将返回：

{
  "eid_number": [
    "cow with this eid number already exists."
  ]
}

之所以有意义，是因为创建了一个新对象。