• Describe the "has many" relationship between Ruby objects.
• Build classes that produce objects with a "belongs-to" and "has-many" relationship.
• Explain why we need to associate objects in this way.

We know that the programs we write are meant to model real-world environments. This is because the programs we write are designed to carry out real-world jobs and solve real-world problems. Whether you're creating an app that connects users around the world in some kind of social network or writing a program for a major university that manages their course offerings and students, your code will need to be able to realistically map the relationships between different entities.

We already know about the "belongs-to" relationship. Let's say we have a Song class that produces individual song objects. Each song belongs to the artist that wrote it. We can build that relationship by creating an attr_accessor in the Song class for artist:

class Song
  attr_accessor :artist, :name, :genre

  def initialize(name, genre)
    @name = name
    @genre = genre
  end
end

If we also have an Artist class that looks like this:

class Artist
  attr_accessor :name

  def initialize(name)
    @name = name
  end
end

We can set an individual instance of Song equal to an instance of the Artist class like this:

kiki = Song.new("In My Feelings", "hip-hop")
drake = Artist.new("Drake")

kiki.artist = drake

kiki.artist.name
  # => "Drake"

The benefit here is that in setting the artist= method equal to a real instance of the Artist class, instead of equal to a simple string, we are associating our song to a robust object that has its own attributes and behaviors.

For example, in the code above, we are calling the #name method on the artist of kiki. With method chaining like this, we can do even more with our code.

The inverse of the "belongs-to" relationship is the "has-many" relationship. If a song belongs to an artist, then an artist should be able to have many songs. This makes sense in the real-world––most musical artists have authored and performed many more than one song.

Let's take a closer look.

How can we represent an object's "having many" of something? Well, having many of something means you own a collection of that thing. Ruby offers us a great way to store collections of data in list form: arrays.

We would like to be able to call:

drake.songs

And have returned to us a list, or array, of the songs that Drake has written. A given artist should start, or be initialized, with a songs collection that is empty. Later, we will write a method that adds songs to that collection.

class Artist
  attr_accessor :name

  def initialize(name)
    @name = name
    @songs = []
  end
end

Here we set an instance variable, @songs, equal to an empty array. Recall that we use instance variables to store the attributes of a given instance of a class. This instance variable is set equal to an empty array because our artist doesn't have any songs yet.

Let's write the method that will allow us to add some.

Whose responsibility is it to add a new song to a given artist's collection? Well, at what point in time does an artist add another song to his or her repertoire? When that artist writes a new song. Consequently, it isn't the song's responsibility to add itself to the artist's collection of songs, it is the artist's responsibility to add a new song to their collection.

That's why we'll write the method that adds songs to an artist's collection in the Artist class:

class Artist
  attr_accessor :name

  def initialize(name)
    @name = name
    @songs = []
  end

  def add_song(song)
    @songs << song
  end
end

Now we can execute the following code:

drake = Artist.new("Drake")
drake.add_song("In My Feelings")
drake.add_song("Hotline Bling")

Now we need a method that will allow a given artist to show us all of the songs in their collection. Let's do it.

Let's write an instance method, #songs, that we can call on an individual artist to return the list of songs that the artist has.

class Artist
  attr_accessor :name

  def initialize(name)
    @name = name
    @songs = []
  end

  def add_song(song)
    @songs << song
  end

  def songs
    @songs
  end
end

The #songs method simply return the @songs array, which contains the list of songs that the artist has many of.

Let's try it out:

drake.songs
  # => ["In My Feelings", "Hotline Bling"]

"Wow, those look like interesting songs," you might be thinking. "I wonder what kind of music Drake makes." Well, let's ask drake to tell us the genres of the songs he has many of.

Oh no! We can't do that because drake's songs are simply a list of strings. We can't ask a plain old string what genre it has—it will have no idea what we are talking about.

This is the limitation of one-sided relationships. Just like associating a given song to a string that contains an artist's name instead of to a real Artist instance had its drawbacks, so too does associating a given artist to a list of strings. With this setup, we are limited to references to a given artist's songs by their name alone. We cannot associate any further information to an artist's songs or enact any further behavior on an artist's songs.

Let's fix this now. Instead of calling the #add_song method with an argument of a string, let's call that method with an argument of a real song object:

kiki = Song.new("In My Feelings", "hip-hop")
hotline = Song.new("Hotline Bling", "pop")

drake.add_song(kiki)
drake.add_song(hotline)

drake.songs
  # =>[#<Song:0x007fa96a878348 @name="In My Feelings", @genre="hip-hop">, #<Song:0x007fa96a122580 @name="Hotline Bling", @genre="pop">]

Great, now our artist has many songs that are real, tangible Song instances, not just strings.

We can do several useful things with this collection of real song objects, such as iterate over them and collect their genres:

drake.songs.collect do |song|
  song.genre
end
  # => ["hip-hop", "pop"]

Now that we can ask our given artist for his songs, let's make sure that we can ask an individual song for its artist:

kiki.artist
  # => nil

Although we do have an attr_accessor for artist in our Song class, this particular song doesn't seem to know that it belongs to Drake. That is because our #add_song method only accomplished associating the song object to the artist object. Our artist knows it has a collection of songs and knows how to add songs to that collection. But, we didn't tell the song that we added to the artist that it belonged to that artist.

Let's fix that now. Telling a song that it belongs to an artist should happen when that song is added to the artist's @songs collection. Consequently, we will write the code that accomplishes this inside our #add_song method:

class Artist
  attr_accessor :name

  def initialize(name)
    @name = name
    @songs = []
  end

  def add_song(song)
    @songs << song
    song.artist = self
  end

  def songs
    @songs
  end
end

Let's take a closer look at the code in our #add_song method:

def add_song(song)
  @songs << song
  song.artist = self
end

Here, we use the self keyword to refer to the artist on which we are calling this method. We call the #artist= method on the song that is being passed in as an argument and set it equal to self––the artist.

Let's try calling #add_song again:

drake.add_song(kiki)

Now, we should be able to ask kiki for its artist:

kiki.artist.name
  # => "Drake"

We did it! Not only does an artist have many songs, but a song belongs to an artist and we built a method that enacts those associations at the appropriate time.

The #add_song method works, but look at it again and see if you might be able to find a flaw in this setup:

def add_song(song)
  @songs << song
  song.artist = self
end

With this implementation, we're maintaining this relationship on both the Song instance and the Artist instance. We've done this so that an artist knows which songs it has, and a song knows the artist it belongs to. However, keeping this information maintained on both sides of the relationship means there are two sources of truth. What happens if we don't consistently use the add_song method? What if, instead, somewhere along the lines we did something like this:

lil_nas_x = Artist.new("Lil Nas X")
old_town_road = Song.new("Old Town Road","hip-hop")

old_town_road.artist = lil_nas_x

old_town_road.artist.name #=> "Lil Nas X"
lil_nas_x.songs #=> []

Now, the Song instance old_town_road is associated with an artist, but lil_nas_x does not know about old_town_road. We have multiple sources of truth about artists and their songs, and they're not aligned.

A better way to approach this would be to figure out how to maintain our "has-many" / "belongs-to" relationship on only one side of the relationship.

Think about it this way - imagine we have many artists, each with their own songs. Rather than have each artist keep track of their own songs, if we had access to a list of all of the songs by all artists, we could just query that list by asking for all songs that belong to a given artist.

This may become clearer if we make some updates to Song and Artist. Say, for instance, in Song, we set up a class variable, @@all, set to an empty Array, and a getter method, .all. This way, when a song is initialized, we can push the instance into the @@all and be able to use Song.all to retrieve all Song instances:

class Song
  attr_accessor :artist, :name, :genre

  @@all = []

  def initialize(name, genre)
    @name = name
    @genre = genre
    save
  end

  def save
    @@all << self
  end

  def self.all
    @@all
  end
end

Okay, so now that we can get all songs, we should be able to do things like this:

lil_nas_x = Artist.new("Lil Nas X")
rick = Artist.new("Rick Astley")

old_town_road = Song.new("Old Town Road","hip-hop")
never_gonna_give_you_up = Song.new("Never Gonna Give You Up","pop")

old_town_road.artist = lil_nas_x
never_gonna_give_you_up.artist = rick

Song.all.first.name #=> "Old Town Road"
Song.all.first.genre #=> "hip-hop"
Song.all.first.artist #=> #<Artist:0x00007ff1d90dbf38 @name="Lil Nas X", @songs=[]>
Song.all.first.artist.name #=> "Lil Nas X"


Song.all.last.name #=> "Never Gonna Give You Up"
Song.all.last.genre #=> "pop"
Song.all.last.artist #=> #<Artist:0x00007ff1d90dbf38 @name="Rick Astley", @songs=[]>
Song.all.last.artist.name #=> "Rick Astley"

Now that we've got a way to get all songs, if we want to find all the songs that belong to a particular artist, we can just select the appropriate songs:

Song.all.select {|song| song.artist == lil_nas_x}
#=> [#<Song:0x00007ff1da1d3228 @name="Old Town Road", @genre="hip-hop", @artist=#<Artist:0x00007ff1d90dbf38 @name="Lil Nas X", @songs=[]>>]

Song.all.select {|song| song.artist == rick}
#=> [#<Song:0x00007ff1da87bc38 @name="Never Gonna Give You Up", @genre="pop", @artist=#<Artist:0x00007ff1da20b150 @name="Rick Astley", @songs=[]>>]

So, with Song.all, if we have an Artist instance like lil_nas_x or rick, we can retrieve all the songs associated with that artist. We can incorporate this directly into our Artist class, replacing the implementation of the #songs method so that it selects instead of returning the @songs instance variable:

class Artist
  ...

  def songs
    Song.all.select {|song| song.artist == self}
  end
end

This is an instance method, so we can use self to represent the Artist instance this method is called on. This changes the rest of the class - if we can just get the necessary information selecting from Song.all, we no longer need the @songs instance variable in our Artist class. We can also update #add_song accordingly:

class Artist
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def add_song(song)
    song.artist = self
  end

  def songs
    Song.all.select {|song| song.artist == self}
  end
end

With this implementation, we're able to achieve a "has-many" / "belongs-to" relationship while maintaining a single source of truth! Not only that, we were able to simplify the Artist class without losing any functionality!

Now, let's go back to the original example in this section. With our new setup, the issue of maintaining both sides of the relationship is solved:

lil_nas_x = Artist.new("Lil Nas X")
old_town_road = Song.new("Old Town Road","hip-hop")

old_town_road.artist = lil_nas_x

old_town_road.artist.name #=> "Lil Nas X"
lil_nas_x.songs #=> [#<Song:0x00007fb46b0a1c08 @name="Old Town Road", @genre="hip-hop", @artist=#<Artist:0x00007fb46b0e3748 @name="Lil Nas X">>]

And add_song functions just as it did before:

rick = Artist.new("Rick Astley")
never_gonna_give_you_up = Song.new("Never Gonna Give You Up","pop")
rick.add_song(never_gonna_give_you_up)

rick.songs #=> [#<Song:0x00007fb46b0b97b8 @name="Never Gonna Give You Up", @genre="pop", @artist=#<Artist:0x00007fb46a903000 @name="Rick Astley">>]
never_gonna_give_you_up.artist #=> #<Artist:0x00007fb46a903000 @name="Rick Astley">

The code we have so far is pretty good. The best thing about it though is that it accommodates future change. We've built solid associations between our Artist and Song class via our "has-many"/"belongs-to" code. With this foundation we can make our code even better in the following ways:

As it currently stands, we have to first create a song and then add it to a given artist's collection of songs. We are lazy programmers, if we could combine these two steps, that would make us happy. Furthermore, if you think about our domain model, i.e. the program we are writing to model the real-world environment of an artist and their songs, the current need to create a song and then add it to an artist doesn't really make sense. A song doesn't exist before an artist creates it.

Instead, let's build a method #add_song_by_name, that takes in an argument of a name and genre and both creates the new song and adds that song to the artist's collection.

class Artist
  ...

  def add_song_by_name(name, genre)
    song = Song.new(name, genre)
    add_song(song)
  end

Here we use the logic of our original #add_song method, which adds a song to an artist's collection and tells that song that it belongs to that particular artist. But, we also create a new song using the name and genre from the arguments.

This is not only neater and more elegant––now we don't have to create a new song on a separate line every time we want to add one to an artist––but it also makes more sense.

Since we've already set up these great associations between instances of the Song and Artist class, we can use them to build other helpful methods.

Currently, to access the name of a given song's artist, we have to chain our methods like this:

kiki.artist.name
  # => "Drake"

We can imagine knowing the name of an artist that a particular song belongs to would be helpful and probably used in mulitple situations. Rather than having to chain multiple methods, wouldn't it be nice if we have one simple and descriptive method that could return the name of a given song's artist? Let's build one!

class Song
  ...

  def artist_name
    self.artist.name
  end

Now we can call:

kiki.artist_name
  # => "Drake"

Much better. Notice that we used the self keyword inside the #artist_name method to refer to the instance of Song on which the method is being called. Then, we call #artist on that song instance. This would return the Artist instance associated with the song. Chaining a call to #name after that is equivalent to saying: call #name on the return value of self.artist, i.e. call #name on the artist of this song.

Using the foundation of a "has-many" / "belongs-to" associations, we can create many different useful methods. We can write methods like add_song_by_name that handle initializing and associating instances. We can also create methods like artist_name, that can simplify retrieving information from an associated instance.

Establishing both "has-many" and "belongs-to" associations between two objects allows us to ask a song who its artist is, and ask an artist what their songs are. We've established a bi-directional relationship! Can you think of any other real world relationships where these associations could be applied?

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