Collection with multiple keys

Question

Hi,

I've seen collections where elements can be accessed using different 'keys' or accessors in an overloaded array sort of way. Let me show you what I mean:

MyCollection[3] would get the 4th object in the collection
MyCollection["title"] would get the object with a title of 'title'
MyCollection[SomeObj] would get the object with the matching SomeObj

I'm not sure how this is done; through some type of extended collection or using some type of interface. Does anyone have any simple examples of doing this? Or any suggestions on what I might be trying to do?

Many thanks,

Rob