緣由： Django 的 ORM 是惰性的每個使用者訪問網站時候，都要進行一次資料互動

在最初開發的時候，需要考慮到使用者量大的時候，
又由於 Django 的 ORM 是惰性的，它只取出當前查詢所需響應最小的資料。它不知道你是否有成百上千的相同或相似的資料也需要取出來。因此訪問網站時這勢必會對資料庫造成巨大壓力！

有句話說的是：只要你在序列化中使用巢狀關係，你就在拿你的效能開玩笑那麼怎麼解決他呢？

解決的方法是使用者訪問時候進行預載入功能，即將資料提前載入到使用者本地中。如何預載入呢？其實django已經提供了方法，在這裡我將工作中常用的載入方式貼出來：

很簡單！在你訪問量大的檢視新增：

queryset = queryset.prefetch_related('你的序列化器名')  

具體列子如下：

class CustomerSerializer(serializers.ModelSerializer):  
    orders = OrderSerializer(many=True, read_only=True)

    def setup_eager_loading(cls, queryset):
        """ Perform necessary eager loading of data. """
        queryset = queryset.prefetch_related('orders'
 
)
        return queryset

呼叫例子：

customer_qs = Customers.objects.all()  
customer_qs = CustomerSerializer.setup_eager_loading(customer_qs)  # Set up eager loading to avoid N+1 selects  
post_data = CustomerSerializer(customer_qs, many=True).data  

以上是簡單實現預載入的原理，我想各位閱讀預載入這塊，程式碼經驗應該還是有的吧？？那我就直接貼出比較具體的程式碼塊了，實現預載入功能！

1. 首先建立一個簡單的資料模型類：

from django.contrib.auth.models import User

class Event:  
    """ A single occasion that has many `attendees` from a number of organizations."""
    creator = models.ForeignKey(User)
    name = models.TextField()
    event_date = models.DateTimeField()

class Attendee:  
    """ A party-goer who (usually) represents an `organization`, who may attend many `events`."""
    events = models.ManyToManyField(Event, related_name='attendees')
    organization = models.ForeignKey(Organization, null=True)

class Organization:  
    name = models.TextField()

2.預載入程式碼如下：

class EventSerializer(serializers.ModelSerializer):  
    creator = serializers.StringRelatedField()
    attendees = AttendeeSerializer(many=True)
    unaffiliated_attendees = AttendeeSerializer(many=True)

    @staticmethod
    def setup_eager_loading(queryset):
        """ Perform necessary eager loading of data. """
        # select_related for "to-one" relationships
        queryset = queryset.select_related('creator')

        # prefetch_related for "to-many" relationships
        queryset = queryset.prefetch_related(
            'attendees',
            'attendees__organization')

        # Prefetch for subsets of relationships
        queryset = queryset.prefetch_related(
            Prefetch('unaffiliated_attendees', 
                queryset=Attendee.objects.filter(organization__isnull=True))
            )
        return queryset

總結：以上是django中實現預載入功能，總之：任何時候你通過 ORM 查詢巢狀關係時，你都應該考慮建立適合的預載入。