how to load 2 related datasets together? (i.e posts and comments) - sql

Question

I'm fairly new to pg and trying to figure out what the best approach is to loading a set of posts and their associated comments together.
For example:
I'm trying to fetch a 10 posts and comments associated with all those posts, like facebooks wall where you see a feed of posts and comments loaded on the same page. My Schema looks something like this:
Posts
--------
id - author - description - date - commentCount
Comments
-------
id - post_id - author - description - date
I tried to load both posts and comments on the same postgres function doing the follow:
select *
from posts
LEFT join comments on posts.id = comments.post_id
unfortunately it duplicated the posts N times where comment exists, where N is the number of comments a post has. However, the first solution is that I can always filter it out in Node after fetching the data
Also when I try to use group by posts.id (to make it easier to traverse in node) I get the following error:
column "comments.id" must appear in the GROUP BY clause or be used in an aggregate function
The second thing I can try is to send an array of post_ids I want to load and have pg_function load and send them back, but I can't quite the query right:
CREATE OR REPLACE FUNCTION "getPosts"(postIds int[])
RETURNS text AS
$BODY$
BEGIN
RETURN (
SELECT *
FROM Comments
WHERE Comments.id = postIds[0]
);
END;$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
to call it:
SELECT n FROM "public"."getPosts"(array[38]) As n;
However, even when trying to get value from one index I get the following error:
ERROR: subquery must return only one column
LINE 1: SELECT (
^
QUERY: SELECT (
SELECT *
FROM Comments
WHERE Comments.id = 38
)
Finally, the last solution is to simple make N seperate calls of postgres, where N is the number of posts with comments, so if I have 5 posts with comments I make 5 calls to postgres with post_id and select from Comments table.
I'm really not sure what to do here, any help would be appreciated.
Thanks

Answer 1

To have all comments as an array of records for each post:
select
p.id, p.title, p.content, p.author,
array_agg(c) as comments
from
posts p
left join
comments c on p.id = c.post_id
group by 1, 2, 3, 4
Or one array for each comment column:
select
p.id, p.title, p.content, p.author,
array_agg(c.author) as comment_author,
array_agg(c.content) as comment_content
from
posts p
left join
comments c on p.id = c.post_id
group by 1, 2, 3, 4