重写Django模型save方法触发唯一约束错误的解决方法

问题情况 #

重写IntoDocumentProduct模型的save方法时，想在创建新对象后基于关联外键赋值，于是在self.pk为None（新对象）时先调用父类的save生成主键，之后再调用一次父类save保存逻辑结果，结果触发IntegrityError，提示主键重复。直接用self.save()又会导致递归循环。

报错信息 #

API测试时的错误 #

IntegrityError at /api/wms/intodocuments/products/create/
duplicate key value violates unique constraint "wms_dokumentprzyjeciaprodukt_pkey"
DETAIL: Key (id)=(151) already exists.

控制台回溯 #

Traceback (most recent call last):
  File "W:\projects\foodgast\venv\Lib\site-packages\django\core\handlers\exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\core\handlers\base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\views\decorators\csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\views\generic\base.py", line 103, in view
    return self.dispatch(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\rest_framework\views.py", line 509, in dispatch
    response = self.handle_exception(exc)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\rest_framework\views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "W:\projects\foodgast\venv\Lib\site-packages\rest_framework\views.py", line 480, in raise_uncaught_exception
    raise exc
  File "W:\projects\foodgast\venv\Lib\site-packages\rest_framework\views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python311\Lib\contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\src\wms\api\views.py", line 183, in post
    serializer.save()
  File "W:\projects\foodgast\venv\Lib\site-packages\rest_framework\serializers.py", line 212, in save
    self.instance = self.create(validated_data)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\rest_framework\serializers.py", line 962, in create
    instance = ModelClass._default_manager.create(**validated_data)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\query.py", line 671, in create
    obj.save(force_insert=True, using=self.db)
  File "W:\projects\foodgast\src\wms\models.py", line 621, in save
    super(IntoDocumentProduct, self).save(*args, **kwargs)
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\base.py", line 812, in save
    self.save_base(
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\base.py", line 863, in save_base
    updated = self._save_table(
              ^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\base.py", line 1006, in _save_table
    results = self._do_insert(
              ^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\base.py", line 1047, in _do_insert
    return manager._insert(
           ^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\query.py", line 1791, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\models\sql\compiler.py", line 1660, in execute_sql
    cursor.execute(sql, params)
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\backends\utils.py", line 103, in execute
    return super().execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\backends\utils.py", line 67, in execute
    return self._execute_with_wrappers(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\backends\utils.py", line 80, in _execute_with_wrappers
    return executor(sql, params, many, context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\backends\utils.py", line 84, in _execute
    with self.db.wrap_database_errors:
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\utils.py", line 91, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "W:\projects\foodgast\venv\Lib\site-packages\django\db\backends\utils.py", line 89, in _execute
    return self.cursor.execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django.db.utils.IntegrityError: duplicate key value violates unique constraint "wms_dokumentprzyjeciaprodukt_pkey"
DETAIL: Key (id)=(150) already exists.

问题根源 #

原代码里，创建新对象时第一次调用父类save已经生成了主键，之后第二次调用父类save时，因为create方法调用save时传入了force_insert=True，Django还是尝试执行插入操作而非更新，导致主键重复冲突。

原问题代码：

def save(self, *args, **kwargs):
        if self.pk is None:
            super(IntoDocumentProduct, self).save(*args, **kwargs)
            # some logic
        # more logic
        super(IntoDocumentProduct, self).save(*args, **kwargs)
        self.full_clean()

解决办法 #

办法1：强制第二次保存为更新操作 #

修改save方法，第一次保存生成主键后，第二次调用父类save时传入force_update=True，告诉Django执行更新而非插入：

def save(self, *args, **kwargs):
    if self.pk is None:
        # 第一次保存生成主键
        super().save(*args, **kwargs)
        # 这里写基于外键的赋值逻辑
        # 比如：self.quantity = self.into_document.default_quantity
    # 通用逻辑放在这里
    # more logic
    # 第二次保存，强制更新操作
    save_kwargs = kwargs.copy()
    if self.pk is not None:
        save_kwargs['force_update'] = True
    super().save(*args, **save_kwargs)
    self.full_clean()

办法2：先拿外键数据再一次性保存 #

如果不需要提前生成主键，可以直接从关联外键对象获取数据赋值，之后只执行一次save，避免两次保存：

def save(self, *args, **kwargs):
    if self.pk is None:
        # 直接从外键对象取数据赋值，不用提前保存
        # 比如：self.price = self.product.retail_price
    # 通用逻辑
    # more logic
    super().save(*args, **kwargs)
    self.full_clean()

办法3：用post_save信号替代重写save方法 #

如果必须在对象创建完成后执行逻辑，推荐用Django的post_save信号，这样不用修改save方法，也不会有递归或重复插入的问题：

from django.db.models.signals import post_save
from django.dispatch import receiver
from .models import IntoDocumentProduct

@receiver(post_save, sender=IntoDocumentProduct)
def update_product_after_create(sender, instance, created, **kwargs):
    if created:
        # 对象刚创建完成，执行逻辑
        # 比如：instance.total = instance.quantity * instance.price
        # 只更新需要修改的字段，避免触发循环
        instance.save(update_fields=['total'])

内容的提问来源于stack exchange，提问作者skelaw