Calculating difference between two dates in rails activerecord select statement

Question

I have a Placements model, which contains check_in and check_out date columns, as well as an integer field 'places'. I'm trying to write a query (postgres) which calculates the total 'person-nights' for a user. A User has many Placements through Listings, so for each placement, I need to find the difference between the check_out and check_in dates in days, and multiply this by the number of places in that placement. Ideally I can do this in one query, something along the lines of the following logic:

  hosted_personnights_count = current_user.hosted_placements
    .select("SUM((placements.check_out - placements.check_in)*placements.people) as datediff")
    .where("placements.confirmed_at IS NOT NULL and placements.check_out < ?", Date.today)
    .group(:user_id)

Obviously this isn't the right syntax, but I thought it was easier to explain this way than try to list all the postgres functions I've tried that don't work. Any ideas? Placements table looks something like:

    create_table "placements", force: true do |t|
     t.integer  "user_id"
     t.integer  "listing_id"
     t.date     "check_in"
     t.date     "check_out"
     t.integer  "people"
     t.datetime "confirmed_at"
     t.datetime "created_at",         null: false
     t.datetime "updated_at",         null: false
    end

Answer 1

Your query is simple as

SELECT SUM((check_out - check_in) * people) AS all
  FROM placements
 WHERE confirmed_at IS NOT NULL
   AND check_out < ?
   AND user_id = ?

You don't need a GROUP BY clause, if you want to sum it all up.

And make sure you don't accidentally select another column (after all ).