Convert SQL to Rails Query

Question

I'm trying to convert the following SQL Query to something Rails can understand:

SELECT posts.* FROM 
(
SELECT group_id, MAX(updated_at) AS updated_at FROM posts WHERE group_id IN (4, 6, 9) GROUP BY group_id
) AS latest_posts 
INNER JOIN posts ON 
latest_posts.group_id = posts.group_id AND 
latest_posts.updated_at = posts.updated_at;

The name of the model is Post . My attempt:

Post.select("(SELECT group_id, MAX(updated_at) AS updated_at FROM posts WHERE group_id IN (4, 6, 9) GROUP BY group_id) AS latest_posts")
.joins('INNER JOIN posts ON latest_posts.group_id = posts.group_id AND latest_posts.updated_at = posts.updated_at')

The error:

Error:
UserTest#test_feed_should_have_the_right_posts:
ActiveRecord::StatementInvalid: PG::DuplicateAlias: ERROR:  table name "posts" specified more than once

    test/models/user_test.rb:100:in `block in <class:UserTest>'

Update: Close but no cigar. Frustrating.

irb(main):046:0> Post.select("posts.* FROM (SELECT group_id, MAX(updated_at) AS updated_at FROM posts WHERE group_id IN (4, 6, 9) GROUP BY group_id) AS latest_posts").joins("INNER J
OIN posts  ON  latest_posts.group_id = posts.group_id  AND  latest_posts.updated_at = posts.updated_at")

Produces:

SELECT posts.* FROM 
(SELECT group_id, MAX(updated_at) AS updated_at FROM posts WHERE group_id IN (4, 6, 9) GROUP BY group_id)
 AS latest_posts 
FROM "posts" 
INNER JOIN posts  
ON  latest_posts.group_id = posts.group_id  AND  latest_posts.updated_at = posts.updated_at

Answer 1

Try

Post.select("(SELECT group_id, MAX(updated_at) AS updated_at FROM posts p1 WHERE group_id IN (4, 6, 9) GROUP BY group_id) AS latest_posts")
.joins('INNER JOIN posts p2 ON latest_posts.group_id = p2.group_id AND latest_posts.updated_at = p2.updated_at')

I suspect that by aliasing the two separate references to the "posts" table, then the query will be acceptable to Rails