Question

I have the following scope, which I know is not optimal:

scope :event_stream_for, lambda{ |user|
  where("target_id in (?) and target_type = ?", user.events.collect(&:id), "Event")
}

This creates 3 queries. How can I optimize it?

Alternatively, how do I put the whole sql statement in lambda of the scope, like

SELECT * FROM activities WHERE target_type =='Event' AND target_id IN (SELECT DISTINCT id FROM events WHERE (host_id == user.id OR invitee_id == user.id))

Thank you

Answer 1

Assuming user has many events and each event belongs to a user.

scope :event_stream_for, lambda{ |user
  joins(:events). # or joins("LEFT JOIN events ON events.user_id = users.id").
  where(["target_type=?", "Event"]) 
}

this will run one query. Haven’t tested my code with your table, but it should work similarly.

———————– based on your edited question ——————-

SELECT * FROM activities WHERE target_type ==’Event’ AND target_id IN (SELECT DISTINCT id FROM events WHERE (host_id == user.id OR invitee_id == user.id))

It’s all about ActiveRecord Relation, http://railscasts.com/episodes/239-activerecord-relation-walkthrough?view=asciicast

Activity.
select("*,distinct events.id AS events_id").
joins("events ON (events.host_id = #{user.id} OR events.invitee_id = #{user.id}").
where(:target_type => 'Event')

Try this on console, and if it works you can just simply change it to scope.
Since i m not sure what you are trying to do, you may need some adjustment.