Performing a complex self-referential query using the Django ORM - sql

Question

I have the following model:
class Message(Model):
url = URLField("URL")
email = EmailField("E-Mail")
contacted = BooleanField("Contacted", default=False)
With example data like:
| url | email | contacted |
+-----+-----------------+-----------+
| foo | foo#example.com | N |
| bar | bar#example.com | N |
| baz | foo#example.com | Y |
I would like to select all distinct rows (by e-mail address) whose e-mail addresses have never been contacted. With this example data, the bar#example.com row would be the only one returned.

Answer 1

This will return the records you want:
not_contacted = Message.objects.exclude(
email__in=Message.objects.filter(contacted=True).values('email')
)
This has the advantage of only running one query. Your query will look something like this:
SELECT
messages_message.id, messages_message.url, messages_message.email, messages_message.contacted
FROM
Messages
WHERE NOT
(messages_message.email IN
( SELECT U0.email from messages_message U0 WHERE U0.contacted = True )
)
Note that for many, many records this query may not be optimal, but it will probably work for most uses.

Answer 2

DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE massage
( zurl varchar NOT NULL
, zemail varchar NOT NULL
, contacted boolean
);
INSERT into massage(zurl, zemail, contacted) VALUES
( 'foo', 'foo#example.com', False)
,( 'bar', 'bar#example.com', False)
,( 'baz', 'foo#example.com', True)
;
SELECT
DISTINCT zemail AS zemail
, MIN(zurl) AS zurl
FROM massage m
WHERE NOT EXISTS (
SELECT *
FROM massage nx
WHERE nx.zemail = m.zemail
AND nx.contacted = True
)
GROUP BY zemail;
If there are multiple records for a given email address, the above one picks the one with the "lowest" URL. If you want them all, the query would be even simpler:
SELECT m.zurl, m.zemail
FROM massage m
WHERE NOT EXISTS (
SELECT *
FROM massage nx
WHERE nx.zemail = m.zemail
AND nx.contacted = True
) ;