python - Django 休息框架 : child object count in serializer

Question

我需要计算一个对象拥有的子对象的数量，并通过对象序列化程序在我的 API 中返回该值。我还需要计算这些子对象的一个​​子集。

我有一个带有子代理人的任务对象。在我的 API 中，当我查询任务时，我希望返回以下数据集:

[
    { label: "Cross the bridge",
      count_assigned: 5,
      count_completed: 3 },
    { label: "Build a fire",
      count_assigned: 5,
      count_completed: 2 }
]

我该怎么做？我找到了 .annotate()方法，但该结果在序列化程序类中不可用。

模型.py

class Task(models.Model):
    label         = models.CharField(max_length=255,null=False,blank=False)

class Assignee(models.model):
    task         = models.ForeignKey(Task, related_name='assignees', on_delete=models.CASCADE, blank=True, null=True) 
    person       = models.ForeignKey(Person, on_delete=models.CASCADE, blank=True, null=True)
    completed    = models.DateTimeField(null=True,blank=True)

序列化器.py

from rest_framework import serializers

from .models import Task, Assignee
from people.serializers import PersonSerializer

class AssigneeSerializer(serializers.ModelSerializer):
    id = serializers.ReadOnlyField()
    person = PersonSerializer(read_only=True)

    class Meta:
        model = Assignee

        fields = ('id','task','person','completed')
        read_only_fields = ['id']


class TaskSerializer(serializers.ModelSerializer):
    id = serializers.ReadOnlyField()

    class Meta:
        model = Task

        fields = ('id', 'label')
        read_only_fields = ['id']

Answer 1

建议的方式

class TaskSerializer(serializers.ModelSerializer):
     id = serializers.ReadOnlyField()
     count_assigned = serializers.SerializerMethodField()
     count_completed = serializers.SerializerMethodField()

     class Meta:
         model = Task
         fields = ('id', 'label', 'count_assigned', 'count_completed')

    def get_count_assigned(self, obj):
        return obj.assignees.count()

    def get_count_completed(self, obj):
        return obj.assignees.exclude(completed__isnull=True).count()

http://www.django-rest-framework.org/api-guide/fields/#serializermethodfield