is extending an object considered acceptable usage?

[mike]

i have an Item which belongs to a Category, so Item has:
- item.categoryId, the database primary key of its Category
- item.category, a reference to its Category.  this null unless i need a 
reference from item to its Category object, in which case i call 
setCategory(category)

sometimes i want a list of categories, and from each i want to be able 
to access a list of its items.  in this case is it considered acceptable 
to just create a list of those items and assign it as a property of 
their category?  eg:

  category.items = listOfItems
this packages everything up into a hierarchy and is more convenient to 
use, especially in Cheetah templates, but requries modifying the 
structure of the object, which bothers me (probably for some 
subconscious java-related reason).

the alternative might be to create a dictionary that keys the lists of 
items on their category:

  items = {}
  items[category.id] = listOfItems
this feels less "controversial" to me, but requires extra objects and 
house-keeping.

thanks - just curious if there were arguments one way or the other.
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