python-3.x - 完整性错误 : FOREIGN KEY constraint failed in django

Question

我有一个 Django View 导致 FOREIGN KEY 约束失败。这来自 related question

def return_book(request,pk):
    books = Books.objects.filter(school = request.user.school).get(id = pk)
    book = Issue.objects.get(book_id_id=pk,book_id__school_id = request.user.school.id)
    user = CustomUser.objects.get(id=request.user.id)
    Return.objects.create(borrowed_item_id=book.id,returner=user)
    Books.Addbook(books) 
    Issue.objects.get(book_id_id=pk).delete()
    return redirect("view_books")

错误返回于 Issue.objects.get(book_id_id=pk).delete()我不知道这个错误的确切原因。有人可以向我解释发生了什么事导致错误>回溯如下。

Traceback (most recent call last):
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\core\handlers\exception.py", line 47, in inner
    response = get_response(request)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\core\handlers\base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\contrib\auth\decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "D:\Python\Django\test projects\library manage\lib_system\Library-System\libman\views.py", line 209, in return_book
    Issue.objects.get(book_id_id=pk).delete()#Borrower_id is also required in the filter
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\models\base.py", line 947, in delete
    return collector.delete()
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\models\deletion.py", line 396, in delete
    count = sql.DeleteQuery(model).delete_batch([instance.pk], self.using)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\models\sql\subqueries.py", line 43, in delete_batch
    num_deleted += self.do_query(self.get_meta().db_table, self.where, using=using)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\models\sql\subqueries.py", line 23, in do_query
    cursor = self.get_compiler(using).execute_sql(CURSOR)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\models\sql\compiler.py", line 1156, in execute_sql
    cursor.execute(sql, params)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\backends\utils.py", line 98, in execute
    return super().execute(sql, params)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\backends\utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\backends\utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\backends\utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\backends\utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "C:\Users\FR GULIK\AppData\Local\Programs\Python\Python38-32\lib\site-packages\django\db\backends\sqlite3\base.py", line 413, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: FOREIGN KEY constraint failed

class Issue(models.Model):
    borrower_id = models.ForeignKey(Student,on_delete=models.CASCADE)
    book_id = models.ForeignKey(Books,on_delete=models.CASCADE)
    issue_date = models.DateField(default=datetime.date.today)
    issuer = models.ForeignKey(CustomUser,on_delete=models.CASCADE)

class Return(models.Model):
    return_date = models.DateField(default=datetime.date.today)
    borrowed_item = models.ForeignKey(Issue,on_delete=models.DO_NOTHING)
    returner = models.ForeignKey(CustomUser,on_delete=models.DO_NOTHING)

Answer 1

Do DO_NOTHING 通常不是一个好的选择，因为(大多数)数据库将检查参照完整性。这意味着他们保证如果 ForeignKey 引用(在本例中为 Issue)，那么这意味着存储问题的表应该有一个主记录Return 项所指的键。

通常 DO_NOTHING 与数据库系统中的某个触发器结合使用(Django 不知道)。

on_delete 最常用的选择通常是:

1. models.CASCADE:在这种情况下，它将删除所有与已删除项相关的Return；
2. models.PROTECT:在这种情况下，它会引发错误以防止删除 Issue；和
3. models.SET_NULL:这是在 NULLable 字段上完成的(因此使用 null=Ture)，在这种情况下，受影响的 Return(s) 会将字段设置为 NULL/None。

另一个选项可能是“软删除”记录，例如 django-soft-delete package [pypi] .在这种情况下，一个 bool 值被添加到 Issue 中，指定该项目是否被删除。该包还将透明地过滤对象，以便您仅检索“有效”的记录。