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I need help with a PostgreSQL query.

I have a Django app + pg backend, where users post interesting URLs (or links) and can then comment under such links (called replies). There are two pg tables to represent this: links_link and links_publicreply.

I am to delete aged rows from links_publicreply. That's easy enough:

DELETE FROM links_publicreply WHERE "submitted_on" < now() - interval '7 days';

But some of these are referenced in the links_link table, hence the query fails. The reference in the links_link table is:

latest_reply = models.ForeignKey
    (
        'links.Publicreply', 
        blank=True, 
        null=True, 
        on_delete=models.SET_NULL
    )

My first question is: How do I delete these foreign key references too?

My hunch is:

begin;
UPDATE links_link SET latest_reply=NULL where latest_reply_id in 
    (SELECT id FROM links_publicreply where "submitted_on" < now() - interval '7 days');
DELETE FROM links_publicreply WHERE "submitted_on" < now() - interval '7 days';
commit;

Does this look right?

If you read the update query above closely, you'll notice it sets latest_reply to null wherever it pointed to an aged links_publicreply row.

I don't actually want to do that. I ideally want to set latest_reply to the next available links_publicreply record (associated to the particular links_link record), which didn't get deleted by the query:

DELETE FROM links_publicreply WHERE "submitted_on" < now() - interval '7 days'; 

How do I do that? That's my second question.


Tables

PG tables, as described in my Django app's models.py, are as follows:

class Link(models.Model):
    description = models.TextField(validators=[MaxLengthValidator(500)])
    submitter = models.ForeignKey(User)
    submitted_on = models.DateTimeField(auto_now_add=True)
    reply_count = models.IntegerField(default=0)
    latest_reply = models.ForeignKey
        ('links.Publicreply', blank=True, null=True, on_delete=models.SET_NULL)

class Publicreply(models.Model):
    submitted_by = models.ForeignKey(User)
    answer_to = models.ForeignKey(Link)
    submitted_on = models.DateTimeField(auto_now_add=True)
    description = models.TextField(validators=[MaxLengthValidator(250)])

1 Answer 1

1

Just add an ON DELETE CASCADE option to your foreign key:

ALTER TABLE links_link 
DROP CONSTRAINT constraint_name,   
ADD CONSTRAINT constraint_name FOREIGN KEY (latest_reply_id)
      REFERENCES links_publicreply(id) ON DELETE CASCADE;

Apparently in django the above is translated as:

on_delete=models.CASCADE
5
  • in Django App he should use configure it within a model, or do extra migration May 1, 2016 at 10:11
  • 1
    @MladenUzelac: If I do it the way Mihai recommends, wouldn't that work as well? Or do I need to do it within the model (or do a migration)? Do add how do add it to the model as an answer. May 1, 2016 at 10:32
  • @HassanBaig I`m not familiar with python,but foreign constraint are a staple of dbs. why would you even use a framework for something that any db does native,probably a lot faster?
    – Mihai
    May 1, 2016 at 10:36
  • Mihai, I prefer it your way, but I might additionally need to update my Django model too, to sync with the change in the DB. Thus asking @MladenUzelac whether there's more to the answer than what you've provided. May 1, 2016 at 10:44
  • I had one project with Django so I wasn't able to give you full answer immediately. And glad that my comment lead to a solution. Working with Django and migrations is elegant way to deal with DB. May 1, 2016 at 15:17

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