Populate Temp Table Postgres - sql

Question

I have the following three tables in the postgres db of my django app:
publication {
id
title
}
tag {
id
title
}
publication_tags{
id
publication_id
tag_id
}
Where tag and publication have a many to many relationship.
I'd like to make a temp table with three columns: 1)publication title, 2)publication id, and 3)tags, where tags is a list (in the form of a string if possible) of all the tags on a given publication.
Thus far I have made the temp table and populated it with the publication id and publication title, but I don't know how to get the tags into it. This is what I have so far:
CREATE TEMP TABLE pubtags (pub_id INTEGER, pub_title VARCHAR(50), pub_tags VARCHAR(50))
INSERT INTO pubtags(pub_id, pub_title) SELECT id, title FROM apricot_app_publication
Can anyone advise me on how I would go about the last step?

Answer 1

Sounds like a job for string_agg:
string_agg(expression, delimiter)
input values concatenated into a string, separated by delimiter
So something like this should do the trick:
insert into pubtags (pub_id, pub_title, pub_tags)
select p.id, p.title, string_agg(t.title, ' ,')
from publication p
join publication_tags pt on (p.id = pt.publication_id)
join tag on (pt.tag_id = t.id)
group by p.id, p.title
You may want to adjust the delimiter, I guessed that a comma would make sense.
I'd recommend using TEXT instead of VARCHAR for your pub_tags so that you don't have to worry about the string aggregation overflowing the pub_tags length. Actually, I'd recommend using TEXT instead of VARCHAR period: PostgreSQL will treat them both the same except for wasting time on length checks with VARCHAR so VARCHAR is pointless unless you have a specific need for a limited length.
Also, if you don't specifically need pub_tags to be a string, you could use an array instead:
CREATE TEMP TABLE pubtags (
pub_id INTEGER,
pub_title TEXT,
pub_tags TEXT[]
)
and array_agg instead of string_agg:
insert into pubtags (pub_id, pub_title, pub_tags)
select p.id, p.title, array_agg(t.title)
-- as above...
Using an array will make it a lot easier to unpack the tags if you need to.