Django 模型多对多反向过滤器

Question

以下是我的模型（类似于）的摘录：

class Person(models.Model):
  name = models.CharField(max_length=20)
  relationships = models.ManyToManyField('self',
    through='Relationship',
    symmetrical=False,
    related_name='related_to',
  )
  def __str__(self):
    return self.name

class Relationship(models.Model):
  from_person = models.ForeignKey(Person,
    related_name='from_people',
    on_delete=models.CASCADE,
  )
  to_person = models.ForeignKey(Person,
    related_name='to_people',
    on_delete=models.CASCADE,
  )
  status = models.CharField(max_length=20)
  def __str__(self):
    return "{} is {} {}".format(
      self.from_person.name, self.status, self.to_person.name)

这是我的数据库的内容：

>>> Person.objects.all()
<QuerySet [<Person: A>, <Person: B>, <Person: C>]>
>>> Relationship.objects.all()
<QuerySet [<Relationship: B is Following C>]>

如果我想查看给定的人正在关注谁，我可以在 Person 中构建一个新方法 class:

def get_following(self):
  return self.relationships.filter(
    to_people__status='Following',
    to_people__from_person=self)

这个有效：

>>> p2.get_following()
<QuerySet [<Person: C>]>

我想做相反的事情。与其问"Who does this person follow?"，不如问"Who follows this person?"。我可以这样做（尽管它是 returns 关系对象，而不是 Person 对象）：

>>> Relationship.objects.filter(to_person=p3, status='Following')
<QuerySet [<Relationship: B is Following to C>]>

我的尝试是这样的（returns 一个空的查询集）：

def get_following(self):
  return self.relationships.filter(
    from_people__status='Following',
    from_people__to_person=self)

感谢您的帮助！

编辑：这是我选择的答案：

def get_followers(self):
  return self.related_to.filter(from_people__status='Following')

Answer 1

查看 this 文档。否则这里有一些其他方法...

self.relationships.from_people.objects.all() 将 return 具有或具有相关名称 from_people 的所有对象。

但是，我会稍微更改一些代码，以便使用 self.relationships.from_people.objects.filter(status='Following')

另一种方法（虽然不是最有效的）是传入人物模型，并使用 person.pk 作为过滤器传入关系模型。

def get_following(self, pk):
    person = Person.objects.get(pk=pk)
    relationships = Relationship.objects.filter(to_person=person.id)
    return relationships

Answer 2

您得到了这样的查询集：<QuerySet [<Relationship: B is Following C>]>。想想有一天（我猜这是提议）'person' 有很多追随者，它可能 return 这么多追随者，像这样：<QuerySet [<Relationship: B is Following C>, <Relationship: A is Following C>]>。所以，我会使用 values_list() [1]:

Relationship.objects.filter(to_person=p3, status='Following').values_list('from_person__name', flat=True)

Returns: <QuerySet [A, B, ...]>

如果只传入单个字段，也可以传入flat参数。如果为 True，这将意味着 returned 结果是单个值，而不是元组。

或创建一个方法：

def get_followers(self):
    follower_of_person = []
    for value in relation_of_person:
        follower_of_p3.append(value.from_person.name)
    return follower_of_person

Returns: [A, B, ...]

values_list 还是更好，因为你直接在数据库上工作。

[1] https://docs.djangoproject.com/en/2.0/ref/models/querysets/#values-list （这里有一个很好的 ManyToMany 例子）。

Answer 3

这是我选择的答案：

def get_followers(self):
  return self.related_to.filter(from_people__status='Following')

Answer 4

以防万一其他人需要另一种实现方法 "Followers" 与您描述的完全一样，但使用不同的建模方案：

# project/account/models.py
from django.db import models
from django.contrib.auth.models import AbstractBaseUser, BaseUserManager, PermissionsMixin
from model_utils import Choices

class User(AbstractBaseUser, PermissionsMixin):
    class Meta:
        # ...

    username = models.CharField(...)
    email = models.EmailField(...)
    # ...

    ### Custom app-specific relationships (database scheme) ###

    # symmetrical=False is needed for this reason: 
    following = models.ManyToManyField('self', related_name='followers', blank=True, symmetrical=False)

# project/account/forms.py
from django import forms
from django.contrib.auth import get_user_model
from django.contrib.admin.widgets import FilteredSelectMultiple
from .models import User

class UserChangeForm(forms.ModelForm):
    # ...

    # new:
    following = forms.ModelMultipleChoiceField(
        queryset=User.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name='Following',
            is_stacked=False
        )
    )
    # new:
    followers = forms.ModelMultipleChoiceField(
        queryset=User.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name='Followers',
            is_stacked=False
        )
    )

    class Meta:
        model = get_user_model()
        # add 'following' and 'followers' to the fields:
        fields = ('email', 'password', ..., 'following', 'followers')

    # also needed to initialize properly:
    def __init__(self, *args, **kwargs):
        super(UserChangeForm, self).__init__(*args, **kwargs)

        # Filter out the self user in the lists and initialize followers list:
        if self.instance and self.instance.pk:
            self.fields['following'] = forms.ModelMultipleChoiceField(
                queryset=User.objects.all().exclude(pk=self.instance.pk),
                required=False,
                widget=FilteredSelectMultiple(
                    verbose_name='Following',
                    is_stacked=False
                )
            )
            self.fields['followers'] = forms.ModelMultipleChoiceField(
                queryset=User.objects.all().exclude(pk=self.instance.pk),
                required=False,
                widget=FilteredSelectMultiple(
                    verbose_name='Followers',
                    is_stacked=False
                )
            )
            self.fields['followers'].initial = self.instance.followers.all()

# project/account/admin.py
from django.contrib.auth import get_user_model
from django.contrib import admin
from django.contrib.auth.models import Group
from django.contrib.auth.admin import UserAdmin as BaseUserAdmin
from .models import User
from .forms import UserChangeForm, UserCreationForm


class Admin(BaseUserAdmin):
    add_form = UserCreationForm
    form = UserChangeForm
    model = get_user_model()

    # The fields to be used in displaying the User model.
    # These override the definitions on the base UserAdmin
    # that reference specific fields on auth.User.
    list_display = ['email', 'username', 'is_admin']
    list_filter = ('is_admin',)
    fieldsets = (
        (None, {'fields': ('email', 'password')}),
        ('Personal info', {'fields': ('username',)}),
        ('Permissions', {'fields': ('is_admin', 'is_superuser', 'is_staff')}),
        # new:
        ('Following / Followers', {'fields': ('following', 'followers')}),
    )
    # other fields
    # ...

    # new:
    filter_horizontal = ('following', 'followers')

admin.site.register(User, Admin)
admin.site.unregister(Group)

如果您随后启动服务器并转到 localhost:8000/admin/ 并导航到用户的详细信息页面，您应该会在屏幕上看到类似这样的内容：

我没有添加计数器，因此您可以立即在 list_view 中看到关注者的数量。

注意第二个带有关注者的 FormField 在管理面板中是只读的。一个用户不能选择其他用户关注他。

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