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 with 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 be building out a domain model for our fictitious music playing app, Playlister. This app will catalog 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 Active Record 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 Active Record association macros will allow our query to get an artist's songs or genres, a song's artist or genre, and a genre's songs and artists entirely through Active Record provided methods on our classes.

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

Open a file, db/migrate/03_create_songs.rb
Write the following migration:

class CreateSongs < ActiveRecord::Migration[4.2]
  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/01_create_artists.rb:

class CreateArtists < ActiveRecord::Migration[4.2]
  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/02_create_genres.rb:

class CreateGenres < ActiveRecord::Migration[4.2]
  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! The tests in this lesson are in place to ensure you've properly set up these associations. You can go ahead and run learn test now to see if you pass them all before continuing.

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! This is great, but we're not quite where we want to be. Right now, we've been able to assign an artist to a song, but is the reverse true?

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

In this case, we still need to tell the adele Artist instance which songs it has. We can do this by pushing the song instance into adele.songs:

[7] pry(main)> adele.songs.push(hello)
=> [#<Song:0x007fc75a8de3d8 id: nil, name: "Hello", artist_id: nil, genre_id: nil>]

Okay, now both sides of the relationships are updated, but so far all the work we've done has been with temporary instances of Artist and Song. To persist these relationships, we can use Active Record's save functionality:

[8] pry(main)> adele.save
=> true
[9] pry(main)> adele
=> #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">

Notice that adele now has an id. What about hello?

[10] pry(main)> hello
=> #<Song:0x007fc75a8de3d8 id: 1, name: "Hello", artist_id: nil, genre_id: nil>

Whoa! We didn't mention hello when we saved. However, we established an association by assigning hello as a song adele has. In order for adele to save, hello must also be saved. Thus, hello has also been given an id.

Go ahead and do the same for hotline_bling and drake to try it out on your own.

Adding Additional Associations

Now, let's make a second song for adele:

[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: 1, name: "Adele">

We've only updated the song, so we should expect that adele is not aware of this song:

[8] pry(main)> someone_like_you.artist
=> #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">
[9] pry(main)> adele.songs
=> [#<Song:0x007fc75b9f3a38 id: 1, name: "Hello", artist_id: 1, genre_id: nil>]

Even if we save the song, adele will not be updated.

[8] pry(main)> someone_like_you.save
=> true
[8] pry(main)> someone_like_you
=> #<Song:0x007fc75b5cabc8 id: 2, name: "Someone Like You", artist_id: 1, genre_id: nil>
[9] pry(main)> adele.songs
=> [#<Song:0x007fc75b9f3a38 id: 1, name: "Hello", artist_id: 1, genre_id: nil>]

But lets see what happens when we switch some things around. Creating one more song:

[8] pry(main)> set_fire_to_the_rain = Song.new(name: "Set Fire to the Rain")
=> #<Song:0x007fc75b5cabc8 id: nil, name: "Set Fire to the Rain", artist_id: nil, genre_id: nil>

Then add the song to adele:

[9] pry(main)> adele.songs.push(set_fire_to_the_rain)
=> [#<Song:0x007fc75b9f3a38 id: 1, name: "Hello", artist_id: 1, genre_id: nil>, #<Song:0x00007feac2be4f38 id: 3, name: "Set Fire to the Rain", artist_id: 1, genre_id: nil>]

Whoa! Check it out - we did not explicitly save set_fire_to_the_rain, but just by pushing the instance into adele.songs, Active Record has gone ahead and saved the instance. Not only that, notice that the song instance also has an aritst_id!

[8] pry(main)> set_fire_to_the_rain.artist
=> #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">

So what is happening? Active Record is doing things for us behind the scenes, but when dealing with associations, it will behave differently depending on which side of a relationship between two models you are updating.

Remember: In a has_many/belongs_to relationship, we can think of the model that has_many as the parent in the relationship. The model that belongs_to, then, is 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 again. 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: 4, name: "Rolling in the Deep", artist_id: 1, genre_id: nil>]
[12] pry(main)> rolling_in_the_deep.artist
=> #<Artist:0x007fc75b8d9490 id: 4, 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. Not only that, rolling_in_the_deep is now persisted to the database.

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, and 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">

This time, we'll just use create directly, which would be the same as running Genre.new, then Genre.save.

[14] pry(main)> pop.songs << rolling_in_the_deep
=> [#<Song:0x007fc75bb4d1e0 id: 4, name: "Rolling in the Deep", artist_id: 1, genre_id: 1>]
[15] pry(main)> pop.songs
=> [#<Song:0x007fc75bb4d1e0 id: 4, name: "Rolling in the Deep", artist_id: 1, genre_id: 1>]
[16] pry(main)> rolling_in_the_deep.genre
=> #<Genre:0x007fa34338d270 id: 1, name: "pop">

It's working! But even cooler is that we've established has many through relationships. By creating a genre, then pushing a song into that genre's list of songs, the genre will now be able to produce its associated artists!

[16] pry(main)> rolling_in_the_deep.artist
=> #<Genre:0x007fa34338d270 id: 1, name: "pop">
[17] pry(main)> pop.artists
=> [#<Artist:0x007fc75b8d9490 id: 1, name: "Adele">]
[28] pry(main)> adele.genres
=> [#<Genre:0x007fa34338d270 id: 1, name: "pop">]

Conclusion

So, order and direction of operations does matter when establishing associations between models - it is typically better to update the has_many side of a relationship to get the full benefit of Active Record's power. Still, as we can see, with just migrations and Active Record macros, we can start to build and persist associations between things!

leplerjacob/active-record-associations-and-migrations