Django REST Serializer做N + 1数据库调用多个嵌套关系,3个级别

Question

我有一种情况,我的模型有外键关系:

# models.py
class Child(models.Model):
    parent = models.ForeignKey(Parent,)

class Parent(models.Model):
    pass

和我的序列化器:

class ParentSerializer(serializer.ModelSerializer):
    child = serializers.SerializerMethodField('get_children_ordered')

    def get_children_ordered(self, parent):
        queryset = Child.objects.filter(parent=parent).select_related('parent')
        serialized_data = ChildSerializer(queryset, many=True, read_only=True, context=self.context)
        return serialized_data.data

    class Meta:
        model = Parent

当我在N个父母的视图中调用Parent时,Django在抓取子节点时在序列化器内进行N次数据库调用.有没有办法让所有的孩子为所有家长最小化数据库呼叫的数量？

我试过这个,但它似乎没有解决我的问题:

class ParentList(generics.ListAPIView):

    def get_queryset(self):
        queryset = Parent.objects.prefetch_related('child')
        return queryset

    serializer_class = ParentSerializer
    permission_classes = (permissions.IsAuthenticated,)

编辑

我已经更新了下面的代码以反映Alex的反馈....它解决了一个嵌套关系的N + 1.

# serializer.py
class ParentSerializer(serializer.ModelSerializer):
    child = serializers.SerializerMethodField('get_children_ordered')

    def get_children_ordered(self, parent):
        # The all() call should hit the cache
        serialized_data = ChildSerializer(parent.child.all(), many=True, read_only=True, context=self.context)
        return serialized_data.data

    class Meta:
            model = Parent

# views.py
class ParentList(generics.ListAPIView):

    def get_queryset(self):
        children = Prefetch('child', queryset=Child.objects.select_related('parent'))
        queryset = Parent.objects.prefetch_related(children)
        return queryset

    serializer_class = ParentSerializer
    permission_classes = (permissions.IsAuthenticated,)

现在让我们说我还有一个模型,这是一个孙子:

# models.py
class GrandChild(models.Model):
    parent = models.ForeignKey(Child,)

class Child(models.Model):
    parent = models.ForeignKey(Parent,)

class Parent(models.Model):
    pass

如果我将以下内容放入我views.py的父母queryset:

queryset = Parent.objects.prefetch_related(children, 'children__grandchildren')

它看起来并不像那些孙子孙女被带入ChildSerializer,因此,我再次运行另一个N + 1问题.对这个有什么想法吗？

编辑2

也许这会提供清晰度......也许我仍然遇到N + 1数据库调用的原因,是因为我的孩子和孙子类都是多态的....即

# models.py
class GrandChild(PolymorphicModel):
    child = models.ForeignKey(Child,)

class GrandSon(GrandChild):
    pass

class GrandDaughter(GrandChild):
    pass

class Child(PolymorphicModel):
    parent = models.ForeignKey(Parent,)

class Son(Child):
    pass

class Daughter(Child):
    pass

class Parent(models.Model):
    pass

我的序列化器看起来更像是这样的:

# serializer.py
class ChildSerializer(serializer.ModelSerializer):
    grandchild = serializers.SerializerMethodField('get_children_ordered')

    def to_representation(self, value):
        if isinstance(value, Son):
            return SonSerializer(value, context=self.context).to_representation(value)
        if isinstance(value, Daughter):
            return DaughterSerializer(value, context=self.context).to_representation(value)

    class Meta:
        model = Child

class ParentSerializer(serializer.ModelSerializer):
    child = serializers.SerializerMethodField('get_children_ordered')

    def get_children_ordered(self, parent):
        queryset = Child.objects.filter(parent=parent).select_related('parent')
        serialized_data = ChildSerializer(queryset, many=True, read_only=True, context=self.context)
        return serialized_data.data

    class Meta:
        model = Parent

再加上Grandaughter,Grandson,我会以代码方式为您提供详细信息,但我认为您可以了解相关信息.

当我为ParentList运行我的视图,并且我监视数据库查询时,我得到的内容就是1000个查询,只有少数父母.

如果我在django shell中运行相同的代码,我可以在不超过25个查询的情况下完成相同的查询.我怀疑它可能与我正在使用django-polymorphic库的事实有关？原因在于,除了每个Son/Daughter,Grandson/Granddaughter表之外,还有一个Child和GrandChild数据库表,共有6个表.跨越这些对象.所以我的直觉告诉我,我错过了那些多态表.

或许对我的daata模型有一个更优雅的解决方案？

Answer 1

据我所知,嵌套序列化程序可以访问预取关系,只需确保不修改查询集(即使用all()):

class ParentSerializer(serializer.ModelSerializer):
    child = serializers.SerializerMethodField('get_children_ordered')

    def get_children_ordered(self, parent):
        # The all() call should hit the cache
        serialized_data = ChildSerializer(parent.child.all(), many=True, read_only=True, context=self.context)
        return serialized_data.data

    class Meta:
            model = Parent


class ParentList(generics.ListAPIView):

    def get_queryset(self):
        children = Prefetch('child', queryset=Child.objects.select_related('parent'))
        queryset = Parent.objects.prefetch_related(children)
        return queryset

    serializer_class = ParentSerializer
    permission_classes = (permissions.IsAuthenticated,)