Distinct Records with joins and order

Question

I have a simple relationship between User and Donations in that a user has many donations, and a donation belongs to a user. What I'd like to do is get a list of users, ordered by the most recent donations.

Here's what I'm trying:

First I want to get the total number of uniq users, which is working as expected:

> User.joins(:donations).order('donations.created_at').uniq.count
   (3.2ms)  SELECT DISTINCT COUNT(DISTINCT "users"."id") FROM "users" INNER JOIN "donations" ON "donations"."user_id" = "users"."id"
 => 384

Next, when I remove the count method, I get an error that "ORDER BY expressions must appear in select list":

> User.joins(:donations).order('donations.created_at').uniq
  User Load (0.9ms)  SELECT DISTINCT "users".* FROM "users" INNER JOIN "donations" ON "donations"."user_id" = "users"."id" ORDER BY donations.created_at
PG::InvalidColumnReference: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ...ON "donations"."user_id" = "users"."id"  ORDER BY donations....

Then I tried fixing the Postgres error by explicitly setting the SELECT clause which at first glance appears to work:

 > User.select('DISTINCT "users".id, "users".*, "donations".created_at').joins(:donations).order('donations.created_at')
  User Load (17.6ms)  SELECT DISTINCT "users".id, "users".*, "donations".created_at FROM "users" INNER JOIN "donations" ON "donations"."user_id" = "users"."id" ORDER BY donations.created_at

However, the number of records returned does not take into account the DISTINCT statement and returns 692 records:

> _.size
 => 692

How do I get the expected number of results (384) while also sorting by the donation's created_at timestamp?

Answer 1

Try this:

User.select('users.*,MAX(donations.created_at) as most_recent_donation').
  joins(:donations).order('most_recent_donation desc').group('users.id')

I suppose an user has many donations, this would select the most recent created donation and would select distinct users filtering by their id.

I have not tested this though.