AR#merge

join_merge.rb

Order.paid.joins(:user).merge(User.blue_eye)

# => SELECT "orders".* FROM "orders" INNER JOIN "users" ON "users"."id" = "orders"."user_id" WHERE "orders"."status" = 3 AND "users"."eye_color" = 'blue' AND "users"."status" = 1

order_in.rb

Order.paid.where(:user_id => User.blue_eye.pluck(:id))

# => SELECT id FROM "users" WHERE "users"."eye_color" = 'blue' AND "users"."status" = 1
# => SELECT "orders".* FROM "orders" WHERE "orders"."status" = 3 AND "orders"."user_id" IN (2, 3)

order_join_v1.rb

Order.paid.joins(:user).where(:users => { :eye_color => 'blue' })

# => SELECT "orders".* FROM "orders" INNER JOIN "users" ON "users"."id" = "orders"."user_id" WHERE "orders"."status" = 3 AND "users"."eye_color" = 'blue'

user_v1.rb

class User < ActiveRecord::Base
  STATUS = { :visible => 1 }
  has_many :orders

  scope :blue_eye, where(:eye_color => 'blue', :status => STATUS[:visible])
end