ActiveRecord Scope Merging

merge.rb

# Merging Scopes
# ==============
# The goal is to join two tables to get all the records where a scope on both
# side of the join is used. I used to do this with a `where()` in which I
# added some sql on the joined table. But, I wanted to use the existing scopes
# from the joining table. Turns out there's a `merge` method on a scope where
# you can merge with another scope without having to chain!

class Car < ActiveRecord::Base
  has_and_belongs_to_many :people
  scope :fueled, -> { where(fueled: true) }
end

class Person < ActiveRecord::Base
  has_and_belongs_to_many :cars
  scope :paid, -> { where(paid: true) }
  
  # get people who have paid and have
  # cars with fuel
  #
  # SELECT "people".*
  # FROM "people"
  #   INNER JOIN "cars_people" ON "cars_people"."person_id" = "people"."id"
  #   INNER JOIN "cars" ON "cars"."id" = "cars_people"."car_id"
  # WHERE "people"."paid" = 't' AND "cars"."fueled" = 't'
  def self.good_to_go
    paid.joins(:cars).merge(Car.fueled)
  end
end