Simplifying a has_many :through (ActiveRecord / Rails). Also a useful boilerplate sharing complex ActiveRecord scenarios.

simplifying_has_many_through.rb

# Run this script with `$ ruby has_much_fun.rb`
require 'sqlite3'
require 'active_record'

# Use `binding.pry` anywhere in this script for easy debugging
require 'pry'

# Connect to an in-memory sqlite3 database
ActiveRecord::Base.establish_connection(
  adapter: 'sqlite3',
  database: ':memory:'
)

# Define a minimal database schema
ActiveRecord::Schema.define do
  create_table :shows do |t|
    t.string :name
  end

  create_table :episodes do |t|
    t.string :name
    t.belongs_to :show, index: true
  end

  create_table :contributions do |t|
    t.belongs_to :episode, index: true
    t.belongs_to :person, index: true
    t.belongs_to :role, index: true
  end

  create_table :people do |t|
    t.string :name
  end

  create_table :roles do |t|
    t.string :name
  end
end

# Define the models
class Show < ActiveRecord::Base
  has_many :episodes, inverse_of: :show
  has_many :contributions, through: :episodes
  has_many :people, through: :contributions

  has_many :actors, -> { joins(:roles).merge(Role.actors) },
    through: :contributions, source: :person
end

class Episode < ActiveRecord::Base
  belongs_to :show, inverse_of: :episodes, required: true
  has_many :contributions, inverse_of: :episode
  has_many :roles, through: :contributions
  has_many :people, through: :contributions
end

class Contribution < ActiveRecord::Base
  belongs_to :episode, inverse_of: :contributions, required: true
  belongs_to :person, inverse_of: :contributions, required: true
  belongs_to :role, inverse_of: :contributions, required: true
end

class Person < ActiveRecord::Base
  has_many :contributions, inverse_of: :person
  has_many :episodes, through: :contributions
  has_many :roles, through: :contributions
end

class Role < ActiveRecord::Base
  has_many :contributions, inverse_of: :role
  has_many :people, through: :contributions
  has_many :episodes, through: :contributions

  scope :actors, -> { where(name: 'Actor') }
end

# Create a few people...
actor_names = [
  'Jim Parsons',
  'Johnny Galecki',
  'Kaley Cuoco',
  'Simon Helberg',
  'Kunal Nayyar'
]

writer_names = [
  'Chuck Lorre',
  'Bill Prady'
]

director_names = [
  'Mark Cendrowski'
]

people_names = actor_names + writer_names + director_names

people_names.each do |person_name|
  Person.create!(name: person_name)
end

# ...and the roles
writer = Role.create!(name: 'Writer')
director = Role.create!(name: 'Director')
actor = Role.create!(name: 'Actor')

# ...and the show and episodes
show = Show.create!(name: 'Big Bang Theory')
episode = show.episodes.create!(name: 'Pilot')

# ... and the contributions.
people_by_role = {
  actor => actor_names,
  writer => writer_names,
  director => director_names
}

people_by_role.each do |role, person_names|
  person_names.each do |person_name|
    person = Person.find_by!(name: person_name)
    Contribution.create!(episode: episode, person: person, role: role)
  end
end

# Now let's find all actors on the show

# Let's try it like this, only using the associations
show.people.joins(:roles).where(roles: { name: 'Actor' }).pluck(:name)
# => ["Jim Parsons", "Johnny Galecki", "Kaley Cuoco", "Simon Helberg", "Kunal Nayyar"]

show.people.joins(:roles).where(roles: { name: 'Actor' }).to_sql
# => 
#  "SELECT \"people\".* FROM \"people\"
#  INNER JOIN \"contributions\" \"contributions_people_join\" ON \"contributions_people_join\".\"person_id\" = \"people\".\"id\"
#  INNER JOIN \"roles\" ON \"roles\".\"id\" = \"contributions_people_join\".\"role_id\"
#  INNER JOIN \"contributions\" ON \"people\".\"id\" = \"contributions\".\"person_id\"
#  INNER JOIN \"episodes\" ON \"contributions\".\"episode_id\" = \"episodes\".\"id\"
#  WHERE \"episodes\".\"show_id\" = 1
#  AND \"roles\".\"name\" = 'Actor'"

show.people.joins(:roles).merge(Role.actors).pluck(:name)
# => ["Jim Parsons", "Johnny Galecki", "Kaley Cuoco", "Simon Helberg", "Kunal Nayyar"]

names_of_actors = show.actors.pluck(:name)
# => ["Jim Parsons", "Johnny Galecki", "Kaley Cuoco", "Simon Helberg", "Kunal Nayyar"]

puts "The actors in #{show.name} are #{ names_of_actors.join(', ') }."
# => The actors in Big Bang Theory are Jim Parsons, Johnny Galecki, Kaley Cuoco, Simon Helberg, Kunal Nayyar.

# Use `binding.pry` here to experiment with this setup.