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

person_belongs_to_exercise.rb

describe Person do
  describe ".in_region" do
    it "returns people in the named region" do
      region = create(:region, name: "expected")
      other_region = create(:region, name: "other")
      in_region = create(:location, region: region)
      in_other_region = create(:location, region: other_region)
      create(:person, location: in_region, name: "in-expected-region-one")
      create(:person, location: in_region, name: "in-expected-region-two")
      create(:person, location: in_other_region, name: "in-other-region")

      result = Person.in_region("expected")

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

#==== SOLUTION ====#
class Person < ActiveRecord::Base
  belongs_to :location
  belongs_to :role
  belongs_to :manager, class_name: "Person", foreign_key: :manager_id
  has_many :employees, class_name: "Person", foreign_key: :manager_id

  def self.in_region(region)
    # all.select { |person| person.location.region.name == region }
    joins(:location).merge(Location.in_region(region))
  end
end

# sql
Person.joins(:location).merge(Location.in_region(region))

SELECT "people".*
FROM "people
 INNER JOIN "locations"
  ON "locations"."id" = "people"."location_id"
LEFT OUTER JOIN "regions"
  ON "regions"."id" = "locations"."region_id"
WHERE "regions"."name" = 'expected'
```

```ruby
describe Person do
  describe ".alphabetically_by_region_and_location" do
    it "orders people by region name, then location name, then person name" do

      region2 = create(:region, name: "region2")
      region3 = create(:region, name: "region3")
      region1 = create(:region, name: "region1")
      location1 = create(:location, name: "location1", region: region2)
      location4 = create(:location, name: "location4", region: region1)
      location3 = create(:location, name: "location3", region: region1)
      location5 = create(:location, name: "location5", region: region3)
      location2 = create(:location, name: "location2", region: region1)
      create(:person, name: "person1", location: location5)
      create(:person, name: "person5", location: location2)
      create(:person, name: "person4", location: location4)
      create(:person, name: "person3", location: location1)
      create(:person, name: "person2", location: location1)
      create(:person, name: "person7", location: location1)
      create(:person, name: "person6", location: location3)

      result = Person.alphabetically_by_region_and_location

      expect(result.map(&:name)).to eq(%w(
        person5
        person6
        person4
        person2
        person3
        person7
        person1
      ))
    end
  end
end

#==== SOLUTION ====#
class Person < ActiveRecord::Base
  belongs_to :location
  belongs_to :role

  # 1st iteration
  def self.alphabetically_by_region_and_location
    Person.all.sort_by do |person|
      [person.location.region.name, person.location.name, person.name]
    end
  end
end

# 2nd iteration
  # after looking here thought :(
  # https://github.com/thoughtbot-upcase-exercises/advanced-activerecord-querying-belongs_to-associations/compare/featured-solution#toc

Person.joins(location: :region).order("regions.name, locations.name, name")

SELECT "people".* FROM "people"
INNER JOIN "locations" ON "locations"."id" = "people"."location_id"
INNER JOIN "regions" ON "regions"."id" = "locations"."region_id"
ORDER BY regions.name, locations.name, name