Django REST 框架 |从多对多字段按计数排序答案

【问题标题】：Django REST Framework | Ordering by Count from a ManyToMany FieldDjango REST 框架 |从多对多字段按计数排序
【发布时间】：2020-11-11 15:07:28
【问题描述】：

我正在学习 Django REST 框架的课程，但我在尝试将 django_filters 中的 ordering 配置设置为 ViewSet 时遇到了困难。

ordering = ('-members__count')

在课程的原始代码中，-members__count 用作返回成员总数的查询，但是当我尝试使用相同的语法时，我得到了这个错误：

错误：

Cannot resolve keyword 'count' into field. Choices are: auth_token, circle, created, date_joined, email, first_name, groups, id, invitation, invited_by, is_active, is_client, is_staff, is_superuser, is_verified, issued_by, last_login, last_name, logentry, membership, modified, password, phone_number, profile, user_permissions, username

也许我误解了这个错误，但所有这些建议的选项都对应于我模型中的字段。此外，在 Course 中，它指出这是一种查询表示格式，用于获取该字段的总数，所以我对此有点困惑。

无论如何，这是我的代码简历，但如果需要，您也可以在此 repo 中查看我的完整详细代码：https://github.com/cadasmeq/comparte_ride/tree/master/cride/circles。

查看：

# Filters
from rest_framework.filters import SearchFilter, OrderingFilter
from django_filters.rest_framework import DjangoFilterBackend

class CircleViewSet(mixins.CreateModelMixin,
                    mixins.RetrieveModelMixin,
                    mixins.UpdateModelMixin,
                    mixins.ListModelMixin,
                    viewsets.GenericViewSet):
    """Circle view set."""

    serializer_class = CircleModelSerializer
    lookup_field = 'slug_name'

    # Filters
    filter_backends = (SearchFilter, OrderingFilter, DjangoFilterBackend)
    search_fields = ('slug_name', 'name')
    ordering_fields = ('rides_offered', 'rides_taken', 'name', 'created', 'member_limit')
    ordering = ('-members__count', '-rides_offered', '-rides_taken')
    filter_fields = ('verified', 'is_limited')

型号：

class Circle(CRideModel):
"""Circle model.

A circle is a private group where rides are offered and taken
by its members. To join a circle a user must receive an unique
invitation code from an existing circle member.
"""

members = models.ManyToManyField(
    "users.User",
    through='circles.Membership',
    through_fields=('circle', 'user')
)

非常感谢大家，对于生疏的英语，我很抱歉哈哈

【问题讨论】：

标签： django django-rest-framework manytomanyfield

【解决方案1】：

您遇到此错误是因为您尝试访问 User 模型的属性。在User 模型上找不到count。我知道您正在尝试按ManyToManyField members 上每个圈子的成员数量排序。

为此，我将使用annotate，将members__count 重命名为members_count 并执行类似（未经测试）的操作：

from django.db.models import Count

class CircleViewSet(mixins.CreateModelMixin,
                    mixins.RetrieveModelMixin,
                    mixins.UpdateModelMixin,
                    mixins.ListModelMixin,
                    viewsets.GenericViewSet):
    """Circle view set."""

    serializer_class = CircleModelSerializer
    lookup_field = 'slug_name'

    # Filters
    filter_backends = (SearchFilter, OrderingFilter, DjangoFilterBackend)
    search_fields = ('slug_name', 'name')
    ordering_fields = ('rides_offered', 'rides_taken', 'name', 'created', 'member_limit')
    ordering = ('-members_count', '-rides_offered', '-rides_taken')
    filter_fields = ('verified', 'is_limited')

    def get_queryset(self):
        """Restrict list to public-only."""
        queryset = Circle.objects.annotate(members_count=Count('members')).all()
        if self.action == 'list':
            return queryset.filter(is_public=True)
        return queryset

【讨论】：