当我在Django中删除模型时，删除了什么？

Question

Lets say I have a model Books:

class Books(models.Model):
    name = models.Charfield()
    author = models.Charfield()

and later I set up another model called BookReviews which links to Books:

class BookReviews(models.Model):
    book = models.ForeignKey(Books)
    content = models.TextField()

But I messed up and I want to delete model BookReviews completely. When I run

python manage.py migrate

I get a warning message:

Any objects related to these content types by a foreign key will also be deleted. Are you sure you want to delete these content types?

Does that mean any entries linked to in Books will also be deleted even if those entries existed before BookReviews?

Answer 1

I think you wonder about on_delete option in ForeignKey

When an object referenced by a ForeignKey is deleted, Django will emulate the behavior of the SQL constraint specified by the on_delete argument. For example, if you have a nullable ForeignKey and you want it to be set null when the referenced object is deleted:

When deleting your Books object, all foreign BookReviews objects deleted because on_delete=models.CASCADE is default. your model is same as below

    class Books(models.Model):
            name = models.Charfield()
            author = models.Charfield()

    class BookReviews(models.Model):
            book = models.ForeignKey(
                    Books,
                    on_delete=models.CASCADE,
            )
            content = models.TextField()

If you don't want automatically delete BookReviews , you can use other on_delete options, like this

    class BookReviews(models.Model):
            book = models.ForeignKey(
                    Books,
                    on_delete=models.SET_NULL,
                    blank=True,
                    null=True,
            )

There's many useful example in django documentation, so check here for more information.

PS after django 2.0 version, you should write on_delete option when writing FroeignKey field.