My notes for https://github.com/thoughtbot-upcase-exercises/advanced-activerecord-querying-belongs_to-associations

location_belongs_to.rb

describe Location do
  describe ".in_region" do
    it "returns locations in the named region" do

      region = create(:region, name: "expected")
      other_region = create(:region, name: "other")
      create(:location, region: region, name: "in-expected-region-one")
      create(:location, region: region, name: "in-expected-region-two")
      create(:location, region: other_region, name: "in-other-region")

      result = Location.in_region("expected")

      expect(result.map(&:name)).
        to match_array(%w(in-expected-region-one in-expected-region-two))
    end
  end
end

#==== SOLUTION ====#
# 1st iteration

class Location < ActiveRecord::Base
  belongs_to :region
  has_many :people

  def self.in_region(region)
    all.select { |location| location.region.name == region }
  end
end

# 2nd iteration

def self.in_region(region)
  all.joins(:region).where(regions: { name: region })
end

# sql

Location.all.joins(:region).where(regions: { name: region })

SELECT "locations".*
FROM "locations"
INNER JOIN "regions"
  ON "regions"."id" = "locations"."region_id"
WHERE "regions"."name" = 'expected'

# 3rd

class Region < ActiveRecord::Base
  has_many :locations

  def self.in_region(region)
    where(name: region)
  end
end


Location.all.joins(:region).merge(Region.in_region(region)).to_sql

SELECT "locations".*
FROM "locations"
INNER JOIN "regions"
  ON "regions"."id" = "locations"."region_id"
WHERE "regions"."name" = 'expected'

# final
class Location < ActiveRecord::Base
  belongs_to :region
  has_many :people

  def self.in_region(region)
    joins(:region).merge(Region.in_region(region))
  end
end