Django更新M2M模型记录触发ValueError：无主键无法强制更新

问题场景 #

我们使用Django定义了多对多中间模型Occurrence，其中attendee和training字段有联合唯一约束，模型及关联模型代码如下：

模型定义 #

# Occurrence 中间模型
class Occurrence(models.Model):
    attendee = models.ForeignKey(Attendee, on_delete=models.CASCADE)
    training = models.ForeignKey(Training, on_delete=models.CASCADE)
    attended_date = models.DateField(default=date(1900, 12, 31))

    class Meta:
        constraints = [
            models.UniqueConstraint(fields=['attendee', 'training'], name='unique_attendee_training')
    ]

# Attendee 模型
class Attendee(models.Model):
    first_name = models.CharField('First Name', max_length=30, blank=False)
    last_name = models.CharField('Last Name', max_length=30, blank=False)
    email = models.EmailField("Email Address", max_length=75, blank=False, primary_key=True)

# Training 模型
class Training(models.Model):
    long_name = models.CharField('Training Name', max_length=80, blank=False, null=False)
    short_name = models.CharField('Abbreviation', max_length=10, blank=False, null=False, primary_key=True)
    description = models.TextField('Descriprion', blank=True)
    alumni = models.ManyToManyField(Attendee, through='Occurrence')

当数据库中已存在某用户的培训记录时，正常查询记录没问题：

from trainings.models import Attendee, Training, Occurrence
from datetime import date, datetime

attendee = Attendee.objects.get(pk='john@gmail.com')
training = Training.objects.get(pk='python310')
occurrence = Occurrence.objects.get(attendee=attendee, training=training)
print(occurrence.attended_date)
# 输出：datetime.date(2021, 11, 4)

但尝试通过新建Occurrence实例并调用save()更新字段时，触发错误：

occurrence = Occurrence(attendee=attendee, training=training, attended_date=date(2021, 11, 5))
occurrence.save(update_fields=["attendee", "training", "attended_date"])

错误信息：

ValueError: Cannot force an update in save() with no primary key.

错误原因 #

新建的Occurrence实例没有主键值，Django无法识别这是一条已存在的记录；而update_fields参数会强制Django执行更新操作，没有主键就找不到要更新的目标记录，因此报错。

正确更新方法 #

方法1：修改已查询到的实例后保存 #

直接使用查询得到的occurrence实例，修改字段后调用save()，还可以指定update_fields只更新需要修改的字段，提升性能：

attendee = Attendee.objects.get(pk='john@gmail.com')
training = Training.objects.get(pk='python310')
occurrence = Occurrence.objects.get(attendee=attendee, training=training)
occurrence.attended_date = date(2021, 11, 5)
# 仅更新attended_date字段，避免不必要的字段写入
occurrence.save(update_fields=["attended_date"])

方法2：使用QuerySet的update()直接更新 #

无需实例化模型，通过QuerySet直接执行更新操作，性能更优（不会触发模型的save()信号）：

attendee = Attendee.objects.get(pk='john@gmail.com')
training = Training.objects.get(pk='python310')
Occurrence.objects.filter(attendee=attendee, training=training).update(attended_date=date(2021, 11, 5))

方法3：用get_or_create()兼容记录存在/不存在场景 #

如果不确定记录是否已存在，可以使用get_or_create()获取实例，再判断是否需要更新：

attendee = Attendee.objects.get(pk='john@gmail.com')
training = Training.objects.get(pk='python310')
occurrence, created = Occurrence.objects.get_or_create(attendee=attendee, training=training)
if not created:
    occurrence.attended_date = date(2021, 11, 5)
    occurrence.save(update_fields=["attended_date"])

内容的提问来源于stack exchange，提问作者Domenico Spidy Tamburro