Cheap debugging of PostgreSQL triggers in Django

Lately, we were hacking on PostgreSQL (PostGIS) triggers, and we quickly felt
like debugging our code... Here is a cheap and quick way of printing out
triggers variables and context through Django.

PostgreSQL server configuration

In postgresql.conf, adjust the minimum level of notice sent to the client :

client_min_messages = log

Note that this does not affect logging verbosity on server.

Catch messages in Django

For a specific model :

from django.db import connection

def save(self, *args, **kwargs):
    before = len(connection.connection.notices)
    try:
        super(Model, self).save(*args, **kwargs)
    finally:
        for notice in connection.connection.notices[before:]:
            print notice

Or globally, using post_save signals (can be verbose):

from django.db import connection
from django.db.models.signals import post_save

def show_notices(sender, instance, created, **kwargs):
    for notice in connection.connection.notices:
        print notice
post_save.connect(show_notices)

Let your trigger be talkative

You can basically print out values, arrays, functions results, records...

RAISE LOG '% has geom %', NEW.id, ST_AsEWKT(NEW.geom);

Will output something like LOG:  3 has geom SRID=4326;POINT(0 0).

FOR record IN SELECT * FROM table
LOOP
    RAISE LOG 'Found %', record;
END LOOP;

Will output something like LOG:  Found (a,b,c).

intersections_on_new := ARRAY[]::float[];
FOR pk IN SELECT ST_Line_Locate_Point(NEW.geom, (ST_Dump(ST_Intersection(other.geom, NEW.geom))).geom)
LOOP
    intersections_on_new := array_append(intersections_on_new, pk);
END LOOP;
RAISE LOG 'Intersects at %', intersections_on_new;

Will output something like LOG:  Intersects at {0.5,0.3}.

One more thing...

If you load your triggers source file through Django (like a post_migrate signal or so),
and thus with psycopg2, you might face that nasty internal quirck :

postgresql_psycopg2/base.py", line 52, in execute
    return self.cursor.execute(query, args)
IndexError: tuple index out of range

This is due to % characters, that you have to escape, replacing them with %%.

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