This article aims to explain one of the fundamental processes of working with databases in Django, deleting. With that, we have to understand the on_delete parameter and its choices.<\/p>\n\n\n\n
So what exactly does the The There are several selection options we can choose from.<\/p>\n\n\n\n Let\u2019s see what each of them does.<\/p>\n\n\n\n What does that mean in practice?<\/p>\n\n\n\n After defining Book and Author models, we have added two Book and two Author instances to the database.<\/p>\n\n\n\n Authors table.<\/strong><\/p>\n\n\n\n Books table.<\/strong><\/p>\n\n\n\n Now, let\u2019s delete our first author.<\/p>\n\n\n\n By the output, we can see that we deleted two model instances.<\/p>\n\n\n\n Let\u2019s see how our database tables look after deletion.<\/p>\n\n\n\n Authors table.<\/strong><\/p>\n\n\n\n Books table.<\/strong><\/p>\n\n\n\n As you can see, all rows connected to the William Shakespeare instance are deleted. This is what Self-explanatory, Let\u2019s see this in action.<\/p>\n\n\n\n Same as before, we will add two books and two authors to the database.<\/p>\n\n\n\n Authors table.<\/strong><\/p>\n\n\n\n Books table.<\/strong><\/p>\n\n\n\n If we try to delete William Shakespeare, let\u2019s see what happens.<\/p>\n\n\n\n All tables data remain the same in the database.<\/p>\n\n\n\n Authors table.<\/strong><\/p>\n\n\n\n Books table.<\/strong><\/p>\n\n\n\n The database data remains the same.<\/p>\n\n\n\non_delete<\/code> parameter do?<\/p>\n\n\n\non_delete<\/code> parameter allows us to define what will happen to the instanced of the model associated with the current instance we are deleting.<\/strong><\/p>\n\n\n\nCASCADE<\/code><\/li>PROTECT<\/code><\/li>RESTRICT<\/code><\/li>SET_NULL<\/code><\/li>SET_DEFAULT<\/code><\/li>SET( )<\/code><\/li>DO_NOTHING<\/code><\/li><\/ul>\n\n\n\nDjango on_delete CASCADE<\/h2>\n\n\n\n
CASCADE<\/code> option deletes any data connected to the instance we are deleting. In other words, it deletes objects containing the foreign key.<\/strong><\/p>\n\n\n\nclass Author(models.Model):\n name = models.CharField(max_length=512)\n\n\nclass Book(models.Model):\n title = models.CharField(max_length=255)\n author = models.ForeignKey(Author, on_delete=models.CASCADE)<\/pre>\n\n\n\n
id<\/strong><\/td> name<\/strong><\/td><\/tr> 1<\/td> William Shakespeare<\/td><\/tr> 2<\/td> Miguel de Cervantes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n id<\/strong><\/td> title<\/strong><\/td> author_id<\/strong><\/td><\/tr> 1<\/td> Romeo and Juliet<\/td> 1<\/td><\/tr> 2<\/td> Don Quixote<\/td> 2<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n >>> Author.objects.get(id=1).delete()\n\n(2, {'Book': 1, 'Author': 1})<\/pre>\n\n\n\nid<\/strong><\/td> name<\/strong><\/td><\/tr> 2<\/td> Miguel de Cervantes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n id<\/strong><\/td> title<\/strong><\/td> author_id<\/strong><\/td><\/tr> 2<\/td> Don Quixote<\/td> 2<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n on_delete=models.CASCADE<\/code> does. If the foreign key model instance is deleted, all related model instances are deleted.<\/strong> This can save us a lot of manual code implementation.<\/p>\n\n\n\nWARNING<\/strong> - be careful with sensitive data.<\/pre>\n\n\n\n
Django on_delete PROTECT<\/h2>\n\n\n\n
PROTECT<\/code> doesn\u2019t allow deletion of instance that is still referenced in some other tables.<\/strong> So if we try to delete an instance that is still referenced, deletion is prevented, and Django raises ProtectedError<\/code>.<\/p>\n\n\n\nclass Author(models.Model):\n name = models.CharField(max_length=512)\n\n\nclass Book(models.Model):\n title = models.CharField(max_length=255)\n author = models.ForeignKey(Author, on_delete=models.PROTECT)<\/pre>\n\n\n\n
id<\/strong><\/td> name<\/strong><\/td><\/tr> 1<\/td> William Shakespeare<\/td><\/tr> 2<\/td> Miguel de Cervantes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n id<\/strong><\/td> title<\/strong><\/td> author_id<\/strong><\/td><\/tr> 1<\/td> Romeo and Juliet<\/td> 1<\/td><\/tr> 2<\/td> Don Quixote<\/td> 2<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n >>> Author.objects.get(id=1).delete()\n\ndjango.db.models.deletion.ProtectedError: (\"Cannot delete some instances of model 'Author' because they are referenced through protected foreign keys: 'Book.author'.\", {<Book: Book object (1)>})\n<\/pre>\n\n\n\nWARNING<\/strong> - you can always manually delete all referenced Books and then Authors, but then you are probably using models.PROTECT option wrong<\/pre>\n\n\n\n
Django on_delete RESTRICT<\/h2>\n\n\n\n
RESTRICT<\/code> works similarly to PROTECT choice, but it raises RestrictedError instead of ProtectedError.<\/strong><\/p>\n\n\n\nclass Author(models.Model):\n name = models.CharField(max_length=512)\n\n\nclass Book(models.Model):\n title = models.CharField(max_length=255)\n author = models.ForeignKey(Author, on_delete=models.RESTRICT)\n<\/pre>\n\n\n\n
id<\/strong><\/td> name<\/strong><\/td><\/tr> 1<\/td> William Shakespeare<\/td><\/tr> 2<\/td> Miguel de Cervantes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n id<\/strong><\/td> title<\/strong><\/td> author_id<\/strong><\/td><\/tr> 1<\/td> Romeo and Juliet<\/td> 1<\/td><\/tr> 2<\/td> Don Quixote<\/td> 2<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n >>>Author.objects.get(id=1).delete()\n\ndjango.db.models.deletion.RestrictedError: (\"Cannot delete some instances of model 'Author' because they are referenced through restricted foreign keys: 'Book.author'.\", {<Book: Book object (1)>})\n<\/pre>\n\n\n\n