There are seven possible actions to take when such event occurs:
CASCADE: When the referenced object is deleted, also delete the objects that have references to it
(when you remove a blog post for instance, you might want to delete comments as well).
SQL equivalent: CASCADE.
PROTECT: Forbid the deletion of the referenced object.
To delete it you will have to delete all objects that reference it manually.
SQL equivalent: RESTRICT.
RESTRICT: (introduced in Django 3.1) Similar behavior as PROTECT that matches SQL's RESTRICT more accurately. (See django documentation example)
SET_NULL: Set the reference to NULL (requires the field to be nullable).
For instance, when you delete a User, you might want to keep the comments he posted on blog posts,
but say it was posted by an anonymous (or deleted) user.
SQL equivalent: SET NULL.
SET_DEFAULT: Set the default value. SQL equivalent: SET DEFAULT.
SET(...): Set a given value. This one is not part of the SQL standard and is entirely handled by Django.
DO_NOTHING: Probably a very bad idea since this would create integrity issues in your database
(referencing an object that actually doesn't exist). SQL equivalent: NO ACTION.