OOP: How would you set up an Artist, Album & Song relationship in OOP?

An album has one or more artist objects. Inheritance would mean an album is an artist. What you need is (EDIT) aggregation: class Album { public $artists = array(); // Array of Artist instances public $songs = array(); // Array of Song instances } Aggregation means every Artist and Song instance may belong to other Albums.

With composition, an artist or song may belong to a single Album.

1 +1 - Exactly correct. – Topher Fangio Jan 8 '10 at 19:07 8 This model doesn't really point out which artists were involved with which songs. I'd think it would be better to have something like class Song { string artists; string name; int genre } along with class Album { Song songs; string catalog; string publisher } – Tom the Junglist Jan 8 '10 at 19:12 +1, probably the clearest way to say what several others (including myself) also said.

– Brian Ensink Jan 8 '10 at 19:12 1 Wouldn't this rather be an aggregate? – Gordon Jan 8 '10 at 19:12 @Gordon : a song might appear on several albums. An artist may participate in several albums.

I don't believe aggregation is adequate. – Victor Nicollet Jan 8 '10 at 19:51.

As I mentioned in one of my comments, if you had to model it via OOP I would say, class Song { string artists; string title; int genre; } class Album { Song tracks; }.

1 I was just going to write this. An album is a collection of Songs and Songs have a title and are performed by Artists. – Gordon Jan 8 '10 at 19:16.

Mmmmm donuts! +1 – Byron Whitlock Jan 8 '10 at 19:08.

Try thinking of it in terms of which objects have which properties. I.e. "The car HAS a color" or "The balloon HAS a string".

Song HAS Artist Album HAS Songs So your Song class would have an Artist object, and your Album class would contain a collection of Song objects.

Artist **has** Songs, in a way that color doesn't 'have' car. In your model does a Song have an Artist, but an Artist doesn't have Songs? If I wanted to know what songs an artist has, how would I express that?

– user151841 Jan 8 '10 at 20:40 I did not completely model the entire structure and what you say is true, Artist has Songs. When you construct your model it would probably make the most sense to have the Song object be the top level entity and each Song would have an Album object and an Artist object.In the case of Album has Songs and Artist has Songs you would perform a search on you base Song collection to find the appropriate tracks for that Artist or Album. Does that make decent enough sense?

– Nathan Taylor Jan 8 '10 at 22:18.

I don't think there is any inheritance in these three classes. Is a Song an Album? No I don't think it is so Song should not inherit from Album.Is an Album an Artist?

No I don't think that is either so Album should not inherit from Artist. Instead you want to look at encapsulation.An Album references zero or more Artists. An Album contains zero ore more songs.

An artist references zero or more Albums.

Inheritance relationship are named "IS-A" while the relationship you're looking for is HAS-A You need something like this: Album artists: Artist1..* songs : Song1..* Where an Album HAS from 1 to many ( * ) artists ( defined by the Artist array ) and 1 album has from 1 to many ( * ) songs, ( defined by the Song array ).

Favor composition over inheritance. By what you said it should be like: An album object has one or more artist objects. An album object has one or more song objects class Album { Artist Artists; Song Songs; } However that's not exactly how I envision this.

I think each Album has one or more songs that is performed by one or more artists. I would do it like: class Album { Songs Songs; // other album specific properties } class Song { Artist Artists; // other song specific properties } class Artist { // artist specific properties } And I highly recommend in looking at OOD principles.

Here's my view. Album contains Song(s) but a Song is not an Album. Album is associated with Artist(s), but an Album is not an Artist.

You are mixing inheritance with relationships.

This sounds like a database modeling question, not one of OOP. This is because you would most likely have a database of songs artists and albums. Not a class hierarchy.

1 I wouldn't say that. There is nothing specific about databases when speaking about inheritance or composition. I think he is just confused on OOP in general.

– Topher Fangio Jan 8 '10 at 19:10 Very true. Especially since ORM's turn your data into objects anyways. – Byron Whitlock Jan 8 '10 at 19:16 There is nothing wrong in having a Domain Model of Artist, Song and Album objects that is ignorant and different from the Persistence Layer, e.g. A database.

– Gordon Jan 8 '10 at 19:52.

I'm not sure you're using OOP correctly. Generally, an object extends a parent when it has the same functionality of that parent, but some extra (or specific) functionality. For example, let's use the auto industry: class Automobile {} class Truck extends Automobile {} class Car extends Automobile {} class Lexus extends Car{} See how Lexus is the most specific, so it builds a hierarchy as it goes up?

What you are wanting is similar to a has_many relationship. An artist has many albums, an album has many songs, etc. Edit 1: As Victor Nicollet said, you want composition. Edit 2: As stated in the comments, you actually want aggregation.

The provided article explains the difference between the two.

It's Aggregation, not Composition. An album does not own the songs. – Gordon Jan 8 '10 at 19:50 A fair explanation, actually.

The OP would have learned that inheritance wasn't relevant, even if this wasn't quite the answer to his question. – David Caunt Jan 8 '10 at 20:00 @Gordon - Good point, I was a bit hasty using the same words other were. Thanks for pointing it out!

– Topher Fangio Jan 8 '10 at 20:06.

If you let us know what you are wishing to accomplish, that may help us arrive at the correct answer. However, these relationships would normally be stored in your database.

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