Question

I have a users table, which has a one-to-many relationship with a user_purchases table via the foreign key user_id. That is, each user can make many purchases (or may have none, in which case he will have no entries in the user_purchases table).

user_purchases has only one other field that is of interest here, which is purchase_date.

I am trying to write a Sequel ORM statement that will return a dataset with the following columns:

• user_id
• date of the users SECOND purchase, if it exists

So users who have not made at least 2 purchases will not appear in this dataset. What is the best way to write this Sequel statement?

Please note I am looking for a dataset with ALL users returned who have >= 2 purchases

Thanks!

EDIT FOR CLARITY

Here is a similar statement I wrote to get users and their first purchase date (as opposed to 2nd purchase date, which I am asking for help with in the current post):

   DB[:users].join(:user_purchases, :user_id => :id)
              .select{[:user_id, min(:purchase_date)]}
              .group(:user_id)

Answer 1

You don’t seem to be worried about the dates, just the counts so

DB[:user_purchases].group_and_count(:user_id).having(:count > 1).all

will return a list of user_ids and counts where the count (of purchases) is >= 2. Something like

[{:count=>2, :user_id=>1}, {:count=>7, :user_id=>2}, {:count=>2, :user_id=>3}, ...]

If you want to get the users with that, the easiest way with Sequel is probably to extract just the list of user_ids and feed that back into another query:

DB[:users].where(:id => DB[:user_purchases].group_and_count(:user_id).
    having(:count > 1).all.map{|row| row[:user_id]}).all

---

Edit:

I felt like there should be a more succinct way and then I saw this answer (from Sequel author Jeremy Evans) to another question using select_group and select_more : https://stackoverflow.com/a/10886982/131226

This should do it without the subselect:

DB[:users].
  left_join(:user_purchases, :user_id=>:id).
  select_group(:id).
  select_more{count(:purchase_date).as(:purchase_count)}.
  having(:purchase_count > 1)

It generates this SQL

SELECT `id`, count(`purchase_date`) AS 'purchase_count' 
FROM `users` LEFT JOIN `user_purchases` 
ON (`user_purchases`.`user_id` = `users`.`id`) 
GROUP BY `id` HAVING (`purchase_count` > 1)"