Integrity_Error (key is still referenced) thrown even with on_delete=do_nothing

Question

I have a model:

class Project(Model):
    name = models.TextField(...)
    ... bunch of fields ...

I also have some models that track history in my application. These models have ForeignKey references to the Project model:

class Historical_Milestone(Model):
    project = models.ForeignKey('Project', db_index=True, related_name='+',
                                on_delete=models.DO_NOTHING, blank=True, null=True)

When I delete an item in my Project table, I get the following IntegrityError:

update or delete on table "project" violates foreign key constraint {...} on table "historical_milestone" DETAIL: Key (id)=(123) is still referenced from table "historical_milestone".

The column in the historical table has related_name='+' set (indicating that I don't want the reverse lookup), and I have the on_delete=models.DO_NOTHING parameter set. According to the documentation on the DO_NOTHING option:

If your database backend enforces referential integrity, this will cause an IntegrityError unless you manually add an SQL ON DELETE constraint to the database field.

Shouldn't the related_name flag indicating I don't care about the reverse relationship take care of this? What manual step do I need to take to modify the Historical_Milestone table to prevent this error from happening?

Answer 1

The related_name is for convenience, it doesn't affect the foreign key constraints that Django creates. Setting related_name='+' simply means you don't want to be able to access product.historical_milestone_set (or similar) in the ORM.

If you want to keep the id after the project is deleted, and not enforce a foreign key constraint, it might be better to use an integer field.

class Historical_Milestone(Model):
    project_id = models.PositiveIntegerField(db_index=True, blank=True, null=True)