Question

I currently have this code:

class Group < ActiveRecord::Base
  has_many :roles

  #can't find better name for this method
  def self.without_role_ids
    find_by_sql('select id from groups where id not in
      (select distinct group_id from roles)').map { |g| g.id }
  end
end

class Role < ActiveRecord::Base
  belongs_to :group
end

without_role_ids method outputs ids of groups that doesn’t have any roles.

What I’m trying to do is to rewrite this method:

def self.without_role_ids
  where("id NOT IN (?)", Role.pluck(:group_id))
end

And it produces two queries.

It is possible to run

where(id: Role.select(:group_id))

and it will produce exactly one SQL query, but with IN instead of NOT IN which I need.

Is it possible to do NOT IN with one query without using find_by_sql?

Answer 1

I found a solution, it is easy to perform such queries with Squeel:

https://github.com/ernie/squeel

Now methods code is:

where{id.not_in Role.select(:group_id)}

and it produces this query:

SELECT "groups".* FROM "groups" WHERE "groups"."id" NOT IN
  (SELECT "roles"."group_id" FROM "roles" )

There is the Railscast about Squeel: http://railscasts.com/episodes/354-squeel