Add note to documentation of how to relate to ProcrastinateJob model from another Django model

[medihack]

When creating a relation from another Django model to the ProcrastinateJob model like this

queued_job = models.OneToOneField(
  ProcrastinateJob, null=True, on_delete=models.SET_NULL, related_name="+"
)

I get an error when jobs are deleted from the database (e.g., by app.run_worker(delete_jobs="always")). This is because Django sets the on_delete trigger only at the application level but not at the database level.

I solved it by altering my table itself with some custom migration. I even created a function for that

from string import Template

def procrastinate_on_delete_sql(app_name: str, model_name: str, reverse=False):
    template = """
        ALTER TABLE ${app_name}_${model_name}
        DROP CONSTRAINT ${app_name}_${model_name}_queued_job_id_key,
        ADD CONSTRAINT ${app_name}_${model_name}_queued_job_id_key
        FOREIGN KEY (queued_job_id) 
        REFERENCES procrastinate_jobs(id) 
        """

    if not reverse:
        template += "\nON DELETE SET NULL;"

    return Template(template).substitute(app_name=app_name, model_name=model_name)

I think we should add a note to the documentation (for example, in the FAQ).

[ewjoachim]

If we document it, I think it makes more sense to document it as a migration and let people factor it differently if relevant for them. For the vast majority of people, the relevant thing to do will be to create a migration.
Also, if you create the FK (or o2o) with db_constraint=False, you don't even need to remove the constraint.

[ewjoachim]

BTW, here's how Django generates the constraint name

[medihack]

If we document it, I think it makes more sense to document it as a migration and let people factor it differently if relevant for them. For the vast majority of people, the relevant thing to do will be to create a migration.

Sure, it was only so that I don't have to look up the SQL later 😉.

Also, if you create the FK (or o2o) with db_constraint=False, you don't even need to remove the constraint.

Thanks, good to know!