Polymorphic associations. Agile rails book confusion.

I think the goal is to get something like CatalogEntry.resource to
accept or return a number of different objects. If polymorphic
associations are being used than we could do something like this:

CatalogEntry.resource = Image.new

or

CatalogEntry.resource = Sound.new

or

CatalogEntry.resource = Video.new

Normal associations won't allow that.

Jim