W4D2 Lighthouse PT Toronto - ActiveRecord Associations

movie.rb

class Movie < ActiveRecord::Base
  has_many :reviews
  has_many :users, through: :reviews
end

rails-c.log

# create an in-memory Movie instance (and throw it away, oops)
2.1.3 :001 > Movie.new
 => #<Movie id: nil, created_at: nil, updated_at: nil> 

# create a persisted Movie object (also capture it in the local variable m)
2.1.3 :002 > m=Movie.create
   (0.2ms)  begin transaction
  SQL (4.7ms)  INSERT INTO "movies" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-04-16 01:09:35.617271"], ["updated_at", "2015-04-16 01:09:35.617271"]]
   (3.2ms)  commit transaction
 => #<Movie id: 1, created_at: "2015-04-16 01:09:35", updated_at: "2015-04-16 01:09:35"> 

# create a persisted User object (capture it in local variable u)
2.1.3 :003 > u=User.create
   (0.0ms)  begin transaction
  SQL (2.9ms)  INSERT INTO "users" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-04-16 01:10:33.593170"], ["updated_at", "2015-04-16 01:10:33.593170"]]
   (2.5ms)  commit transaction
 => #<User id: 1, created_at: "2015-04-16 01:10:33", updated_at: "2015-04-16 01:10:33"> 

# create a persisted review but fail to capture it (note the id in the output to use in the next step)
2.1.3 :004 > Review.create
   (0.1ms)  begin transaction
  SQL (4.5ms)  INSERT INTO "reviews" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-04-16 01:10:46.450161"], ["updated_at", "2015-04-16 01:10:46.450161"]]
   (3.5ms)  commit transaction
 => #<Review id: 1, created_at: "2015-04-16 01:10:46", updated_at: "2015-04-16 01:10:46", user_id: nil, movie_id: nil> 

# retrieve the persisted Review object from the database
2.1.3 :005 > r=Review.find(1)
  Review Load (1.5ms)  SELECT  "reviews".* FROM "reviews"  WHERE "reviews"."id" = ? LIMIT 1  [["id", 1]]
 => #<Review id: 1, created_at: "2015-04-16 01:10:46", updated_at: "2015-04-16 01:10:46", user_id: nil, movie_id: nil> 

# ** USING THE AUTO-GENERATED (by ActiveRecord) ASSOCIATION ACCESSOR METHODS TO MODIFY ASSOCIATIONS **
# set the review's user to the instance in the variable u
2.1.3 :006 > r.user = u
 => #<User id: 1, created_at: "2015-04-16 01:10:33", updated_at: "2015-04-16 01:10:33"> 
# set the reviews's movie to the instance in the variable m
2.1.3 :007 > r.movie = m
 => #<Movie id: 1, created_at: "2015-04-16 01:09:35", updated_at: "2015-04-16 01:09:35"> 
# a complete Review! But it's not saved yet.
2.1.3 :008 > r
 => #<Review id: 1, created_at: "2015-04-16 01:10:46", updated_at: "2015-04-16 01:10:46", user_id: 1, movie_id: 1> 
2.1.3 :009 > r.save
   (0.4ms)  begin transaction
  SQL (3.5ms)  UPDATE "reviews" SET "movie_id" = ?, "updated_at" = ?, "user_id" = ? WHERE "reviews"."id" = 1  [["movie_id", 1], ["updated_at", "2015-04-16 01:11:44.023846"], ["user_id", 1]]
   (3.0ms)  commit transaction
 => true 
# here's our movie
2.1.3 :010 > m
 => #<Movie id: 1, created_at: "2015-04-16 01:09:35", updated_at: "2015-04-16 01:09:35"> 
# traverse the association to find it's review...
2.1.3 :011 > m.review 
NoMethodError: undefined method `review' for #<Movie:0xc0d413c>
# acutally, it's reviews (one-to-many)
2.1.3 :012 > m.reviews
  Review Load (0.9ms)  SELECT "reviews".* FROM "reviews"  WHERE "reviews"."movie_id" = ?  [["movie_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Review id: 1, created_at: "2015-04-16 01:10:46", updated_at: "2015-04-16 01:11:44", user_id: 1, movie_id: 1>]> 
# now traverse the has_many through relationship...
2.1.3 :013 > m.users  
  User Load (0.9ms)  SELECT "users".* FROM "users" INNER JOIN "reviews" ON "users"."id" = "reviews"."user_id" WHERE "reviews"."movie_id" = ?  [["movie_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<User id: 1, created_at: "2015-04-16 01:10:33", updated_at: "2015-04-16 01:10:33">]> 

# similarly traverse the associations from the User side...
2.1.3 :014 > u.reviews
  Review Load (1.5ms)  SELECT "reviews".* FROM "reviews"  WHERE "reviews"."user_id" = ?  [["user_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Review id: 1, created_at: "2015-04-16 01:10:46", updated_at: "2015-04-16 01:11:44", user_id: 1, movie_id: 1>]> 
2.1.3 :015 > u.movies 
  Movie Load (1.6ms)  SELECT "movies".* FROM "movies" INNER JOIN "reviews" ON "movies"."id" = "reviews"."movie_id" WHERE "reviews"."user_id" = ?  [["user_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Movie id: 1, created_at: "2015-04-16 01:09:35", updated_at: "2015-04-16 01:09:35">]> 

# answer the question: how many movies has this user reviewed?
# this is the "hard" way since it uses the more complicated has_many through association
2.1.3 :016 > u.movies.count
   (1.6ms)  SELECT COUNT(*) FROM "movies" INNER JOIN "reviews" ON "movies"."id" = "reviews"."movie_id" WHERE "reviews"."user_id" = ?  [["user_id", 1]]
 => 1 
# using the knowledge that a User can only review each Movie once, this will always return the same result with less work
2.1.3 :017 > u.reviews.count
   (1.3ms)  SELECT COUNT(*) FROM "reviews"  WHERE "reviews"."user_id" = ?  [["user_id", 1]]
 => 1 

# from the review, we can now get to the movie without hitting the database!
2.1.3 :021 > r.movie
 => #<Movie id: 1, created_at: "2015-04-16 01:09:35", updated_at: "2015-04-16 01:09:35"> 

# create some new objects... a new User and Movie
2.1.3 :022 > u2=User.create
   (0.1ms)  begin transaction
  SQL (3.2ms)  INSERT INTO "users" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-04-16 01:14:40.519086"], ["updated_at", "2015-04-16 01:14:40.519086"]]
   (2.2ms)  commit transaction
 => #<User id: 2, created_at: "2015-04-16 01:14:40", updated_at: "2015-04-16 01:14:40"> 
2.1.3 :023 > m2=Movie.create
   (0.1ms)  begin transaction
  SQL (4.2ms)  INSERT INTO "movies" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2015-04-16 01:14:48.565140"], ["updated_at", "2015-04-16 01:14:48.565140"]]
   (2.8ms)  commit transaction
 => #<Movie id: 2, created_at: "2015-04-16 01:14:48", updated_at: "2015-04-16 01:14:48"> 

# create a review for the second user, but for the first movie 
2.1.3 :024 > r2=Review.create(user: u2, movie: m)
   (0.1ms)  begin transaction
  SQL (3.4ms)  INSERT INTO "reviews" ("created_at", "movie_id", "updated_at", "user_id") VALUES (?, ?, ?, ?)  [["created_at", "2015-04-16 01:15:17.359864"], ["movie_id", 1], ["updated_at", "2015-04-16 01:15:17.359864"], ["user_id", 2]]
   (3.6ms)  commit transaction
 => #<Review id: 2, created_at: "2015-04-16 01:15:17", updated_at: "2015-04-16 01:15:17", user_id: 2, movie_id: 1> 

# answer the question: how many reviews are there for the first Movie (the one stored in m)
2.1.3 :027 > m.reviews.count
   (1.9ms)  SELECT COUNT(*) FROM "reviews"  WHERE "reviews"."movie_id" = ?  [["movie_id", 1]]
 => 2 

# answer the question: which users have reviewed the first Movie (the one stored in m)
2.1.3 :028 > m.users        
 => #<ActiveRecord::Associations::CollectionProxy [#<User id: 1, created_at: "2015-04-16 01:10:33", updated_at: "2015-04-16 01:10:33">]> 

# answer the question: how many users have reviewed the first Movie (note: same as number of reviews)
2.1.3 :029 > m.users.count
   (2.1ms)  SELECT COUNT(*) FROM "users" INNER JOIN "reviews" ON "users"."id" = "reviews"."user_id" WHERE "reviews"."movie_id" = ?  [["movie_id", 1]]
 => 2 

# answer the question: which users have reviewed the second Movie (the one stored in the variable m2)
2.1.3 :036 > m2.users
  User Load (1.3ms)  SELECT "users".* FROM "users" INNER JOIN "reviews" ON "users"."id" = "reviews"."user_id" WHERE "reviews"."movie_id" = ?  [["movie_id", 2]]
 => #<ActiveRecord::Associations::CollectionProxy []> 

# answer the question: how many users have reviewed the second movie (same as the number of reviews, just harder to compute)
2.1.3 :037 > m2.users.count
   (2.5ms)  SELECT COUNT(*) FROM "users" INNER JOIN "reviews" ON "users"."id" = "reviews"."user_id" WHERE "reviews"."movie_id" = ?  [["movie_id", 2]]
 => 0 

# retrieve the reviews for the second Movie (there are none!)
2.1.3 :038 > m2.reviews    
  Review Load (1.8ms)  SELECT "reviews".* FROM "reviews"  WHERE "reviews"."movie_id" = ?  [["movie_id", 2]]
 => #<ActiveRecord::Associations::CollectionProxy []> 

review.rb

class Review < ActiveRecord::Base
  belongs_to :movie
  belongs_to :user
end

user.rb

class User < ActiveRecord::Base
  has_many :reviews
  has_many :movies, through: :reviews
end