Finyasy/phase-3-active-record-associations-methods

Active Record Association Methods

Learning Goals

• Understand the common methods we have access to from our Active Record associations
• Use the methods that Active Record gives you based on your associations

Introduction

Previously, we learned what Active Record associations are and how to use them. In this lab, we are going to start with the association relationships already coded for Songs, Genres, and Artists. These associations look like this:

• An artist has many songs
• An artist has many genres through songs.
• A song belongs to a genre.
• A song belongs to a artist.
• A genre has many songs.
• A genre has many artists through songs.

Here's what our Entity Relationship Diagram looks like:

In the previous lessons, we had a similar set of associations between games, reviews and users, where the reviews table joined between our users and games.

While the models are different for this lab, our approach will be the same. By writing a few migrations and making use of the appropriate Active Record macros, 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

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

Building our Migrations

You can take a look at the migrations to see how our tables are structured. Pay special attention to the migration for the songs table — this is the table that joins between our artists and genres tables.

To run the migrations, run:

$ bundle exec rake db:migrate

Building our Associations using Active Record Macros

In the previous lesson, we used the following Active Record macros (or methods): has_many, has_many through, belongs_to. They helped us associate games, reviews, and users.

Here's how they look for our new domain:

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

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

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

And that's it! With this relatively small amount of code, we now have access to a whole host of methods provided by Active Record.

Association Methods

Go ahead and run the test suite and you'll see that we are passing the first 14 tests. Our associations are all working, just because of our migrations and use of macros.

We can now call methods on the objects we associated with one another. Let's play around with our code using the console task we wrote for you in the Rakefile. Run the migrations (if you haven't already):

$ bundle exec rake db:migrate

Then seed the database:

$ bundle exec rake db:seed

Then open the console:

$ bundle exec rake console

Then try out some methods:

hello = Song.create(name: "Hello")
# => #<Song:0x007fc75a8de3d8 id: 1, name: "Hello", artist_id: nil, genre_id: nil>
adele = Artist.create(name: "Adele")
# => #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">

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:

hello.artist = adele
# => #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">

Now, we can ask hello who its artist is:

hello.artist
# => #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">

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

hello.artist.name
# => "Adele"

We can tell the artist about their song:

rolling_in_the_deep = Song.create(name: "Rolling in the Deep")
# => #<Song:0x007fc75bb4d1e0 id: 2, name: "Rolling in the Deep", artist_id: nil, genre_id: nil>

adele.songs << rolling_in_the_deep
# => #[ <Song:0x007fc75bb4d1e0 id: 2, name: "Rolling in the Deep", artist_id: 1, genre_id: nil> ]

rolling_in_the_deep.artist
# => #<Artist:0x007fc75b8d9490 id: 1, name: "Adele">

Starting the Lab

We are going to write some methods of our own. We want to take advantage of the instance methods provided by the Active Record macros. Therefore, every method we write will use some code that was generated by a macro. For example:

class Artist
  has_many :songs
  has_many :genres, through: :songs

  def get_first_song

  end
end

How would you write the #get_first_song method so that it returns the first song saved to the artist it's called on? By using the macros! Just like above when we called adele.songs, we now want to call #songs on the instance that the method will be called on in the future. How do we do that? Yes, self!

class Artist
  has_many :songs
  has_many :genres, through: :songs

  def get_first_song
    self.songs
  end
end

Recall that using has_many :songs creates an instance method #songs that we can call on any instance of an artist.

Calling this method now will return an array of the artist's songs. Since our method is specifically looking for the first song, we just have to chain on a #first.

class Artist
  has_many :songs
  has_many :genres, through: :songs

  def get_first_song
    self.songs.first
  end
end

We'll do a handful of methods like this one for the Song, Artist, and Genre classes. This lab is test-driven, so follow the specs and read the test error messages for additional information.

The below methods are defined in the artist.rb, genre.rb and song.rb files within app/models, but are all currently empty. Write implementations for each using ActiveRecord methods.

Artist Methods

#get_genre_of_first_song

Returns the genre of the artist's first saved song (maybe the #get_first_song method can be used here?).

#song_count

Return the total number of songs associated with the artist.

#genre_count

Return the total number of genres associated with the artist.

Genre Methods

#song_count

Return the total number of songs associated with the genre.

#artist_count

Return the number of artists associated with the genre.

#all_artist_names

Return an array of strings containing every artist's name.

Song Methods

#get_genre_name

Return the name of the genre this song belongs to.

#drake_made_this

For the final method in this lab, rather than return a specific value or set of values like the previous labs, your task is to create an association between a song and an artist. In this case, we'll use one artist for simplicity — Drake.

When this method is called, it should assign the song's artist to Drake. Drake doesn't exist in the database as an artist yet, so you'll have to create a record for him. However, if this method is run multiple times, you won't want to create a new record each time. Rather, you only want to create a record if Drake is not found in the database already. Once found or created, assign this song to the drake Artist instance.

Hint: Look into the .find_or_create_by Active Record method!