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I have seen elsewhere that it is a nice design to implement links between objects in a database as a separate object. This makes a clean way to have multiple objects of one class to be linked to an object of another class, as opposed to adding a "links" property to an object, which then gets populated with, say a CSV list of other objects that are linked.

Now I want to go through my music collection (MP3) and find differently encoded versions of the same recording (via audio fingerprinting - think of the same song ripped twice with a different bit rate for example). Now I am not sure how to design the link objects, since they link objects of the same class. I could just do something like

id songA songB

but then if I want to search for all matches for a particular song, I will have to go through both songA and songB to find possible links. Is there a smarter way to do this?

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you are talking about the 5th Normal Form... I suggest you read up on all of them, and learn when which is required en.wikipedia.org/wiki/Fifth_normal_form –  AK_ Feb 6 at 16:20
    
Also worth pointing out that relationship database design is very different to object orientated design. Using table records and objects interchangeably is incorrect. A lot of web frameworks confuse the matter by having a table record hold the data for a single object, but this is a convenience, it shouldn't be understood as relational databases being anything to do with objects and classes. –  Cormac Mulhall Feb 10 at 13:44
    
Can you recommend any further ressources on that matter? –  Isaac Feb 10 at 15:00

1 Answer 1

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This doesn't call for a linking entity (although they very useful for N:M relations and infinitely better than trying to cram a list of things into a single field or multiple fields!).

What you want is a SONG entity that can be referenced by multiple RECORDING entities. A SONG has a title, artist etc., while a RECORDING might have a bit rate, loudness, file date etc. The RECORDING would have a single foreign key to the SONG table. That's how you express N:1 associations.

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That is definitely smarter, thanks. I will have to think about how I create a SONG entity from a recording that might be incorrectly tagged, but that should be doable. Maybe the SONG just needs an id anyway. –  Isaac Feb 6 at 17:10

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