Active Record Associations

Rails supports six types of associations:

• belongs_to
• has_one
• has_many
• has_many :through
• has_one :through
• has_and_belongs_to_many

Database Layer

This is a data view of one table:

This is a schema view of all the tables:

Model Layer

Let's work with the models in a 1 to M relationship

user = User.create(username: 'chip')
(0.1ms)  begin transaction
SQL (7.1ms)  INSERT INTO "users" ("created_at", "updated_at", "username") VALUES (?, ?, ?)  [["created_at", Mon, 23 Mar 2015 20:39:39 UTC +00:00], ["updated_at", Mon, 23 Mar 2015 20:39:39 UTC +00:00], ["username", "chip"]]
(1.8ms)  commit transaction
 => #<User id: 6, username: "chip", created_at: "2015-03-23 20:39:39", updated_at: "2015-03-23 20:39:39">

post = user.posts.create(title: 'Rails')
(1.2ms)  begin transaction
SQL (5.7ms)  INSERT INTO "posts" ("created_at", "title", "updated_at", "user_id") VALUES (?, ?, ?, ?)  [["created_at", Mon, 23 Mar 2015 21:05:21 UTC +00:00], ["title", "Rails"], ["updated_at", Mon, 23 Mar 2015 21:05:21 UTC +00:00], ["user_id", 6]]
(1.6ms)  commit transaction
 => #<Post id: 5, title: "Rails", url: nil, description: nil, created_at: "2015-03-23 21:05:21", updated_at: "2015-03-23 21:05:21", user_id: 6>

user.posts << post
(0.1ms)  begin transaction
(0.1ms)  commit transaction
 => #<ActiveRecord::Associations::CollectionProxy [#<Post id: 4, title: "Rails for dummies", url: nil, description: nil, created_at: "2015-03-23 20:45:05", updated_at: "2015-03-23 20:45:05", user_id: 6>, #<Post id: 5, title: "Rails", url: nil, description: nil, created_at: "2015-03-23 21:05:21", updated_at: "2015-03-23 21:05:21", user_id: 6>, #<Post id: 5, title: "Rails", url: nil, description: nil, created_at: "2015-03-23 21:05:21", updated_at: "2015-03-23 21:05:21", user_id: 6>]>

post.save
(0.1ms)  begin transaction
(0.1ms)  commit transaction
 => true

post.creator.username
 => "chip"

user.posts
 => #<ActiveRecord::Associations::CollectionProxy [#<Post id: 4, title: "Rails for dummies", url: nil, description: nil, created_at: "2015-03-23 20:45:05", updated_at: "2015-03-23 20:45:05", user_id: 6>, #<Post id: 5, title: "Rails", url: nil, description: nil, created_at: "2015-03-23 21:05:21", updated_at: "2015-03-23 21:05:21", user_id: 6>]>

 

Let's work with the models in a M to M through relationship

post = Post.find 1
Post Load (8.6ms)  SELECT "posts".* FROM "posts" WHERE "posts"."id" = ? LIMIT 1  [["id", 1]]
=> <#Post id: 1, title: "Test Post", url: nil, description: "Description 1", created_at: "2015-03-18 23:13:21", updated_at: "2015-03-19 23:54:06", user_id: 5>

post.categories
Category Load (0.2ms)  SELECT "categories".* FROM "categories" INNER JOIN "post_categories" ON "categories"."id" = "post_categories"."category_id" WHERE "post_categories"."post_id" = ?  [["post_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Category id: 1, name: "Python", created_at: "2015-03-18 23:26:21", updated_at: "2015-03-18 23:26:21">]>

cat = Category.create(name: "Ruby Books")
(0.2ms)  begin transaction
SQL (6.8ms)  INSERT INTO "categories" ("created_at" "name", "updated_at") VALUES (?, ?, ?)  [["created_at", Sun, 22 Mar 2015 08:40:06 UTC +00:00], ["name", "Ruby Books"], ["updated_at", Sun, 22 Mar 2015 08:40:06 UTC +00:00]]
(1.0ms)  commit transaction
 => #<Category id: 2, name: "Ruby Books", created_at: "2015-03-22 08:40:06", updated_at: "2015-03-22 08:40:06"%gt;

post.categories << cat
(0.1ms)  begin transaction
SQL (0.6ms)  INSERT INTO "post_categories" ("category_id", "created_at", "post_id", "updated_at") VALUES (?, ?, ?, ?)  [["category_id", 2], ["created_at", Sun, 22 Mar 2015 08:40:38 UTC +00:00], ["post_id", 1], ["updated_at", Sun, 22 Mar 2015 08:40:38 UTC +00:00]]
(0.8ms)  commit transaction
 => #<ActiveRecord::Associations::CollectionProxy [#<Category id: 1, name: "Python", created_at: "2015-03-18 23:26:21", updated_at: "2015-03-18 23:26:21">, #<Category id: 2, name: "Ruby Books", created_at: "2015-03-22 08:40:06", updated_at: "2015-03-22 08:40:06">]>

cat.name
 => "Ruby Books"