我目前有这个要序列化的 Django 模型:
class Result(models.Model):
...
routes = models.ManyToManyField(Route)
...
class Route(models.Model):
...
class Feature(models.Model):
result = models.ForeignKey(Result)
route = models.ForeignKey(Route)
description = models.TextField()
DRF 序列化器看起来像:
class ResultSerializer(serializers.ModelSerializer):
...
route = RouteSerializer(many=True, required=False)
...
class Meta:
model = Result
fields = '__all__'
class FeatureField(serializers.CharField):
"""
Accepts text in the writes and looks up the correct feature for the reads.
"""
def get_attribute(self, obj):
# We pass the object instance onto `to_representation`, not just the field attribute.
return obj
def to_representation(self, obj):
try:
search_result = self.root.child.instance
# FIXME: this is the problem.
feature = Feature.objects.get(route=obj.id, search_result=search_result)
feature = feature.description
except Feature.DoesNotExist:
feature = None
return feature
class RouteSerializer(serializers.ModelSerializer):
description = FeatureField(required=False)
class Meta:
model = Route
fields = '__all__'
我在代码中的意思是，当我使用只有一个实例的 ResultSerializer 时，这会起作用，但是如果我想在 ListView 中序列化多个实例，并且我将一个查询集传递给序列化程序， DRF 在其上应用了一个 ListSerializer，现在 self.root.instance 是一个记录列表，我无法访问调用嵌套 RouteSerializer 的单个结果，因此我无法检索到正确的特征。
请您参考如下方法:
我跳进了 DRF 代码，终于明白是怎么回事了:
如果您使用 serializer = ResultSerializer(result) 只序列化一个实例，则 serializer.instance 只包含这个单一的、特定的 result 实例，并且嵌套的序列化程序和字段可以使用 self.root.instance 毫无问题地访问它。
现在，如果您序列化多个实例，就像默认的 list 操作一样，实际发生的情况如下:
- 执行类似
serializer = ResultSerializer(queryset, many=True)的调用 - 参数中有
many=True会触发BaseSerializer的many_init()方法，这会创建一个ResultSerializer以查询集作为实例，所以serializer.instance是查询集。 - 接下来它创建一个扩展
ResultSerializer的单个ListSerializer并且它的实例再次是查询集。
我想错了，认为 ListSerializer 会为查询集中的每个元素创建单独的 ResultSerializer。
我最终解决这个问题的方法是重写 ResultSerializer.to_representation() 方法:
class ResultSerializer(serializers.ModelSerializer):
def to_representation(self, instance):
# When we call Results with many=True, the serializer.instance is a list with several records,
# we can't know which particular instance is spawning the nested serializers so we add it here.
self._instance = instance
return super(ResultSerializer, self).to_representation(instance)
最后像这样在 FeatureField 中使用它:
class FeatureField(serializers.CharField):
"""
Accepts text in the writes and looks up the correct feature for the reads.
"""
def get_attribute(self, obj):
# We pass the object instance onto `to_representation`, not just the field attribute.
return obj
def to_representation(self, obj):
# If the root is a ListSerializer, retrieve the right Result instance using the `_instance` attribute.
try:
if isinstance(self.root, serializers.ListSerializer):
search_result = self.root.child._instance
else:
search_result = self.root.instance
feature = Feature.objects.get(route=obj.id, search_result=search_result)
feature = feature.pickup_instructions
except Feature.DoesNotExist:
feature = None
return feature
