Django 检查ManyToMany 字段中是否有对象

在Django中，我们经常使用ManyToMany字段来建立模型之间的多对多关系。这种字段允许一个模型实例关联多个其他模型实例，而且这些关联是互相独立的。但有时候我们需要检查一个ManyToMany字段中是否有对象，以便在逻辑中做出相应的处理。

python
from django.db import models
class User(models.Model):
    name = models.CharField(max_length=100)
    groups = models.ManyToManyField('Group', related_name='members')
    
class Group(models.Model):
    name = models.CharField(max_length=100)

python
group = Group.objects.get(name='Group1')
has_members = group.members.exists()
if has_members:
    print("This group has members.")
else:
    print("This group doesn't have any members.")

python
from django.db.models import Count
groups = Group.objects.annotate(num_members=Count('members'))
for group in groups:
    if group.num_members > 0:
        print(f"The group {group.name} has members.")
    else:
        print(f"The group {group.name} doesn't have any members.")

python
user = User.objects.get(name='John')
belong_to_group = user.groups.filter(members=user).exists()
if belong_to_group:
    print("This user belongs to at least one group.")
else:
    print("This user doesn't belong to any group.")