IntegrityError當刪除模型實例

Question

我有兩個模型，如

class A(models.Model): 
    title = models.CharField(max_length=255) 

class B(models.Model): 
    recommendation = models.ForeignKey(A, related_name="+") 
    title = models.CharField(max_length=255) 

當我刪除了一個模型的實例，我得到的是這樣的：

IntegrityError: update or delete on table "myapp_a" violates foreign key constraint "myapp_relate_recommendation_id_4a7c5340_fk_myapp_a_id" on table "myapp_b" 
Detail: Key (id)=(27527) is still referenced from table "myapp_b". 

我想不通爲什麼它會發生，我認爲FK應該被默認刪除。

Answer 1

嘗試添加on_delete參數傳送給recommendation領域：

models.py

class A(models.Model): 
    title = models.CharField(max_length=255) 

class B(models.Model): 
    recommendation = models.ForeignKey(A, related_name="+", on_delete=models.CASCADE) 
    title = models.CharField(max_length=255) 

運行python manage.py shell

>>> from abtest.models import A, B 
>>> a = A(title="test A title") 
>>> a.save() 
>>> a 
<A: A object (1)> 
>>> b = B(title="test B title", recommendation=a) 
>>> b.save() 
>>> b 
<B: B object (1)> 
>>> b.recommendation 
<A: A object (1)> 
>>> a.delete() 
(2, {'abtest.B': 1, 'abtest.A': 1}) 

您可以瞭解更多關於Django的如何處理外來鍵刪除here。