Selecting requests a user hasn't responded to yet

Question

I'm using heroku/ rails/ pgsql 9.1.6 and banging my head against the wall around what feels like it should be a really simple query.

I want to surface recent "Requests" that a user hasn't responded to yet

Attempt 1 (works before I push to heroku/ pgsql):

Request.includes(:responses).where("(select count(responses.id)=0 WHERE responses.user=?) AND requests.created_at > ?", user_id, days_ago ) 

Once I push to heroku, that query generates an error:

ActiveRecord::StatementInvalid: PGError: ERROR: aggregates not allowed in WHERE clause

Attempt 2: "Having"

I found a thread on StackOverflow saying I should try "Having" instead, so I revised to

Request.includes(:responses).having("responses.user_id=? AND count(responses.id)=0", user_id).where("requests.created_at > ?", days_ago )

That generated a new error. ActiveRecord::StatementInvalid: PGError: ERROR:

column "requests.id" must appear in the GROUP BY clause or be used in an aggregate function

Attempt 3: "Group"

I then revised that query to

Request.includes(:responses).group("requests.id").group("responses.id").having("responses.user_id=? AND count(responses.id)=0", user_id).where("requests.created_at > ?", days_ago ) 

This runs, but returns an empty set even when I know it should return a value.

Best Guess:

I suspect the problem at this point is with the 2 part having clause, and that in the current format it's inadvertently excluding users who haven't responded (and thus will always be an empty set):

e.g. having("responses.user_id=? AND count(responses.id)=0", user_id),

as opposed to the original embedded select:

select count(responses.id)=0 WHERE responses.user=?

but I'm not sure what the right form for the having clause should be to surface users who have not responded.

Thanks in advance for any help!

Answer 1

try

Request.where('(SELECT Count(*) FROM responses WHERE responses.request_id = requests.id) > 0')