Question

I have User and Gift models. A user can send gifts to another users. I have a relational table telling me which users received a gift. On the other hand, a user belongs to a School, which can be free or paid.

I want the count of users that have received a gift in the last week for a specific type of school (this is, free or paid).

I can do:

Gift.joins(:schools).where("created_at >= ? AND schools.free_school = ?", Time.now.beggining_of_week, true).collect(&:gift_recipients).flatten.uniq.count.

Or, I want to know how many users sent gifts the last week. This works:

Gift.joins(:schools).where("created_at >= ? AND schools.free_school = ?", Time.now.beggining_of_week, true).collect(&:user_id).uniq.count.

If I want to know how many users have sent or received a gift in the last week I can do:

(Gift.joins(:schools).where("created_at >= ? AND schools.free_school = ?", Time.now.beggining_of_week, true).collect(&:gift_recipients).flatten + Gift.joins(:schools).where("created_at >= ? AND schools.free_school = ?", Time.now.beggining_of_week, true).collect(&:user_id)).uniq.count

All this works fine but if the database is big enough this is really slow. Do you have any suggestions to make it more efficient, maybe using raw SQL where needed?

"gifts" 
  user_id (integer) 
  school_id (integer) 
  created_at (datetime) 
  updated_at (datetime) 
"gift_recipients" is a table like 
  gift_id | recipient_id,

Answer 1

You do not want to do this using collect(), which is loading all of the results into memory and filtering them within an Array of ActiveRecords. This is slow and dangerous, as it could potential leak/use all of the memory available, depending on the size of the data vs. your server.

Once you post your schema I can help you query/aggregate this in SQL, which is the right way to do it.

For example, instead of:

Gift.joins(:schools).where("created_at >= ? AND schools.free_school = ?", Time.now.beggining_of_week, true).collect(&:user_id).uniq.count

You should use:

Gift.joins(:schools).where("created_at >= ? AND schools.free_school = ?", Time.now.beggining_of_week, true).count('distinct user_id')

…which will count the distinct user_ids in SQL and return the result instead of returning all of the objects and counting them in memory.