Postgresql – Tricky condition for PostgreSQL UPDATE query

cascadeforeign keypostgresqlpostgresql-9.3

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)])

Best Answer

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

Related Solutions

PostgreSQL – insert/update violates foreign key constraints

There are a few problems with your tables. I'll try to address the foreign keys first, since you question asked about them :)

But before that, we should realize that the two sets of tables (the first three you created and the second set, which you created after dropping the first set) are the same. Of course, the definition of Table3 in your second attempt has syntax and logical errors, but the basic idea is:

CREATE TABLE table3 (   
  "ID" bigint NOT NULL DEFAULT '0',   
  "DataID" bigint DEFAULT NULL,   
  "Address" numeric(20) DEFAULT NULL,   
  "Data" bigint DEFAULT NULL,
   PRIMARY KEY ("ID"),   
   FOREIGN KEY ("DataID") REFERENCES Table1("DataID") on delete cascade on update cascade,   
   FOREIGN KEY ("Address") REFERENCES Table2("Address") on delete cascade on update cascade
);

This definition tell PostgreSQL roughly the following: "Create a table with four columns, one will be the primary key (PK), the others can be NULL. If a new row is inserted, check DataID and Address: if they contain a non-NULL value (say 27856), then check Table1 for DataID˙and Table2 for Address. If there is no such value in those tables, then return an error." This last point which you've seen first:

ERROR: insert or update on table "Table3" violates foreign key constraint 
    "Table3_DataID_fkey" DETAIL: Key (DataID)=(27856) is not present in table "Table1".

So simple: if there is no row in Table1 where DataID = 27856, then you can't insert that row into Table3.

If you need that row, you should first insert a row into Table1 with DataID = 27856, and only then try to insert into Table3. If this seems to you not what you want, please describe in a few sentences what you want to achieve, and we can help with a good design.

And now about the other problems.

You define your PKs as

CREATE all_your_tables (
    first_column NOT NULL DEFAULT '0',   
    [...]
    PRIMARY KEY ("ID"),

A primary key means that all the items in it are different from each other, that is, the values are UNIQUE. If you give a static DEFAULT (like '0') to a UNIQUE column, you will experience bad surprises all the time. This is what you got in your third error message.

Furthermore, '0' means a text string, but not a number (bigint or numeric in your case). Use simply 0 instead (or don't use it at all, as I written above).

And a last point (I may be wrong here): in Table2, your Address field is set to numeric(20). At the same time, it is the PK of the table. The column name and the data type suggests that this address can change in the future. If this is true, than it is a very bad choice for a PK. Think about the following scenario: you have an address '1234567890454', which has a child in Table3 like

ID        DataID           Address             Data
123       3216547          1234567890454       654897564134569

Now that address happens to change to something other. How do you make your child row in Table3 follow its parent to the new address? (There are solutions for this, but can cause much confusion.) If this is your case, add an ID column to your table, which will not contain any information from the real world, it will simply serve as an identification value (that is, ID) for an address.

Mysql – Foreign key set to cascading but fails to update – why

It appears as if this is not permitted in innodb which I assume is the engine you are using. From http://dev.mysql.com/doc/refman/5.6/en/innodb-foreign-key-constraints.html:

If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the cascade, it acts like RESTRICT. This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the other hand, is possible, as is a self-referential ON DELETE CASCADE. Cascading operations may not be nested more than 15 levels deep.

Tables

Best Answer

Related Solutions

PostgreSQL – insert/update violates foreign key constraints

Mysql – Foreign key set to cascading but fails to update – why

Related Question