Active Record Associations Introduction: Playlister

Objectives

Understand how and why Active Record implements associations between models.
Use Active Record migrations and methods to build out a domain model that associates classes via the has-many/belongs-to and the many-to-many (or has-many-through) relationships.

What are Active Record Associations?

We already know that we can build our classes such that they associate to one another. We also know that it takes a lot of code to do it. Active Record associations allow us to associate models and their analogous database tables without having to write tons of code.

Additionally, Active Record associations make actually working with our associated objects even quicker, neater and easier.

Sounds great, right? Now that we have you totally hooked, let's take a look at how we use these AR associations.

How do we use AR Associations?

Active Record makes it easy to implement the following relationships between models:

belongs_to
has_one
has_many
has_many :through
has_one :through
has_and_belongs_to_many

We don't need to worry about most of these right now. We'll concern ourselves with relationships that should sound familiar:

belongs to
has many
has many through

In order to implement these relationships we will need to do two things:

Write a migration that creates tables with associations. For example, if a cat belongs to an owner, the cats table should have an owner_id column.
Use Active Record macros in the models.

We'll go through both of these steps together, using our Playlister domain model.

Overview

In this walk-through, we'll build building out a domain model for our fictitious music playing app, Playlister. This app will catalogue songs and their associated artists and genres.

We'll have three models: Artists, Songs, and Genres. By writing a few migrations and making use of the appropriate ActiveRecord macros (more on that later), we will be able to:

ask an Artist about its songs and genres
ask a Song about its genre and its artist
ask a Genre about its songs and artists.

The relationships between artists, songs and genres will be enacted as follows:

Artists have many songs and a song belongs to an artist.
Artists have many genres through songs.
Songs belong to a genre.
A genre has many songs.
A genre has many artists through songs.

We will build these associations through the use of Active Record migrations and macros.

Building our Migrations

The Song model

A song will belong to an artist and belong to a genre. Before we worry about the migration that will implement this in our songs table, let's think about what that table will look like:

id	name	artist_id	genre_id
2	Shake It Off	1	1

We can see that the songs table will have an artist_id column and a genre_id column. We will give a given song an artist_id value of the artist it belongs to. The same goes for genre. These foreign keys, in conjunction with the ActiveRecord association macros will allow us query to get an artist's songs or genres, a song's artist or genre, and a genre's songs and artists entirely through ActiveRecord provided methods on our classes.

Let's write the migration that will make this happen.

Create a file, db/migrations/001_create_song_table.rb
Write the following migration:

class CreateSongs < ActiveRecord::Migration
  def change
    create_table :songs do |t|
      t.string :name 
      t.integer :artist_id
      t.integer :genre_id
    end
  end
end

The Artist Model

An artist will have many songs and it will have many genres through songs. These associations will be taken care of entirely through AR macros, which we'll get to in a bit. What do we mean by through songs? The table songs is the JOIN table! Remember that from previous labs? That means that songs has both an artist_id and a genre_id to combine those two tables together in a many to many relationship.

Let's take a look at what our artists table will need to look like:

id	name
1	Taylor Swift

Our artists table just needs a name column. Let's write the migration. In db/migrate/002_create_artists_table.rb:

class CreateArtists < ActiveRecord::Migration
  def change
    create_table :artists do |t|
      t.string :name
    end
  end
end

The Genre Model

A genre will have many songs and it will have many artists through songs. These associations will be taken care of entirely through AR macros, which we'll get to in a bit.

Let's take a look at what our genres table will need to look like:

id	name
1	pop

Let's write our migration. In db/migrate/003_create_genres_table.rb:

class CreateGenres < ActiveRecord::Migration
  def change
    create_table :genres do |t|
      t.string :name 
    end
  end
end

Great! Now go ahead and run rake db:migrate in your terminal to execute our table creations.

Building our Associations using AR Macros

What is a macro?

A macro is a method that writes code for us (think metaprogramming). By invoking a few methods that come with Active Record, we can implement all of the associations we've been discussing.

We'll be using the following AR macros (or methods):

Let's get started.

A Song Belongs to an Artist and A Genre

Create a file, app/models/song.rb. Define your Song class to inherit from ActiveRecord::Base. This is very important! If we don't inherit from Active Record Base, we won't get our fancy macro methods.

class Song < ActiveRecord::Base

end

We need to tell the Song class that it will produce objects that can belong to an artist. We will do it with the belongs_to macro:

class Song < ActiveRecord::Base
  belongs_to :artist
end

Songs also belong to a genre, so we'll use the same macro to implement that relationship:

class Song < ActiveRecord::Base
  belongs_to :artist
  belongs_to :genre
end

An Artist Has Many Songs

Create a file, app/models/artist.rb. Define your Artist class to inherit from ActiveRecord::Base:

class Artist < ActiveRecord::Base

end

We need to tell the Artist class that each artist object can have many songs. We will use the has_many macro to do it.

class Artist < ActiveRecord::Base
  has_many :songs

end

And that's it! Now, because our songs table has an artist_id column and because our Artist class uses the has_many macro, an artist has many songs!

It is also true that an artist has many genres through songs. We will use the has_many through macro to implement this:

class Artist < ActiveRecord::Base
  has_many :songs
  has_many :genres, through: :songs
end

Genres Have Many Songs and Have Many Artists

Create a file app/models/genre.rb. In it, define a class, Genre, to inherit from ActiveRecord::Base.

class Genre < ActiveRecord::Base

end

A genre can have many songs. Let's implement that with the has_many macro:

class Genre < ActiveRecord::Base
  has_many :songs
end

A genre also has many artists through its songs. Let's implement this relationship with the has_many through macro:

class Genre < ActiveRecord::Base
  has_many :songs
  has_many :artists, through: :songs
end

And that's it!

Our Code in Action: Working with Associations

Go ahead and run the test suite and you'll see that we are passing all of our tests! Amazing! Our associations are all working, just because of our migrations and use of macros.

Let's play around with our code.

In your console, run rake console. Now we are in a Pry console that accesses our models.

Let's make a few new songs:

[1]pry(main)> hello = Song.new(name: "Hello")
=> #<Song:0x007fc75a8de3d8 id: nil, name: "Hello", artist_id: nil, genre_id: nil>
[2]pry(main)> hotline_bling = Song.new(name: "Hotline Bling")
=> #<Song:0x007fc75b9f3a38 id: nil, name: "Hotline Bling", artist_id: nil, genre_id: nil>

Okay, here we have two songs. Let's make some artists to associate them to. In the same PRY sessions as above:

[3] pry(main)> adele = Artist.new(name: "Adele")
=> #<Artist:0x007fc75b8d9490 id: nil, name: "Adele">
[4] pry(main)> drake = Artist.new(name: "Drake")
=> #<Artist:0x007fc75b163c60 id: nil, name: "Drake">

So, we know that an individual song has an artist_id attribute. We could associate hello to adele by setting hello.artist_id= equal to the id of the adele object. BUT! Active Record makes it so easy for us. The macros we implemented in our classes allow us to associate a song object directly to an artist object:

[5] pry(main)> hello.artist = adele
=> #<Artist:0x007fc75b8d9490 id: nil, name: "Adele">

Now, we can ask hello who its artist is:

[6] pry(main)> hello.artist
=> #<Artist:0x007fc75b8d9490 id: nil, name: "Adele">

We can even chain methods to ask hello for the name of its artist:

[7] pry(main)> hello.artist.name
=> "Adele"

Wow!

Go ahead and do the same for hotline_bling and drake.

We can also ask our artists what songs they have. Let's make a second song for adele first:

[8] pry(main)> someone_like_you = Song.new(name: "Someone Like You")
=> #<Song:0x007fc75b5cabc8 id: nil, name: "Someone Like You", artist_id: nil, genre_id: nil>
[8] pry(main)> someone_like_you.artist = adele
=> #<Artist:0x007fc75b8d9490 id: nil, name: "Adele">

Now let's ask adele for her songs:

[9] pry(main)> adele.songs
=> []

Huh? How can adele's collection of songs be empty? We associated two songs with adele! Here's the thing, and this is important to remember:

The model that has_many is considered the parent. The model that belongs_to is considered the child. If you tell the child that it belongs to the parent, the parent won't know about that relationship. If you tell the parent that a certain child object has been added to its collection, both the parent and the child will know about the association.

Let's see this in action. Let's create another new song and add it to adele's songs collection:

[10] pry(main)> rolling_in_the_deep = Song.new(name: "Rolling in the Deep")
=> #<Song:0x007fc75bb4d1e0 id: nil, name: "Rolling in the Deep", artist_id: nil, genre_id: nil>

[11] pry(main)> adele.songs << rolling_in_the_deep
=> [ #<Song:0x007fc75bb4d1e0 id: nil, name: "Rolling in the Deep", artist_id: nil, genre_id: nil>]
[12] pry(main)> rolling_in_the_deep.artist
=> #<Artist:0x007fc75b8d9490 id: nil, name: "Adele">

We added rolling_in_the_deep to adele's collection of songs and we can see the adele knows it has that song in the collection and rolling_in_the_deep knows about its artist.

Notice that adele.songs returns an array of songs. When a model has_many of something, it will store those objects in an array. To add to that collection, we use the shovel operator, <<, to operate on that collection, treat adele.songs like any other array.

Let's play around with some genres and our has many through association.

[13] pry(main)> pop = Genre.create(name: "pop")
=> #<Genre:0x007fa34338d270 id: 1, name: "pop">

[14] pry(main)> pop.songs << rolling_in_the_deep
=> [#<Song:0x007fc75bb4d1e0 id: nil, name: "Rolling in the Deep", artist_id: nil, genre_id: nil>]
[15] pry(main)> pop.songs
=> [#<Song:0x007fc75bb4d1e0 id: nil, name: "Rolling in the Deep", artist_id: nil, genre_id: nil>]
[16] pry(main)> rolling_in_the_deep.genre
=> #<Genre:0x007fa34338d270 id: 1, name: "pop">
[17] pry(main)> pop.artists
=> [#<Artist:0x007fa342e34dc8 id: 1, name: "Adele">]

It's working!

RMcNeely/activerecord-associations-intro-v-000