counter_cache has_many_through sql optimisation, reduce number of sql queries

Question

How I can optimise my SQL queries, to ignore situations like this:

Meeting.find(5).users.size => SELECT COUNT(*) FROM ... WHERE ...

User.find(1

Answer 1

As far as I know you can't use counter_cache with through associations, that's why you should manually increment it.

For example (untested):

class MeetingUser < ActiveRecord::Base

  ...

  after_create { |record| 
    Meeting.increment_counter(:users_count, record.meeting.id)
  }

  after_destroy { |record| 
    Meeting.decrement_counter(:users_count, record.meeting.id)
  }

end