[elephant-devel] Newbie question: Index on collections

[Ian Eslick]

The pset is probably your best bet.  Other collection objects have  
more performance impact.  With a pset you can ask:

is X a friend of Y?  (find-item X (friends Y)) => X | nil

all people that X is friends of:

(remove-nulls
   (map-class 'people (lambda (y)
                         (when (find-item X (friends Y))
                            Y))
              :collect t))

This requires walking all person objects.  For larger DB's, you can  
build your own 'many-to-many' relation table or waiting for the next  
release which should do this for you.

(defpclass friend-relation ()
    ((first :accessor first :initarg :first :index t)
     (second :accessor second :initarg :second :index t)))

(defmethod add-friend-relation (x y)
    (make-instance 'friend-relation :first x :second y))

(defmethod friends-of (x)
    (union (mapcar #'second (get-instances-by-slot 'friend-relation  
'first x))
           (mapcar #'first (get-instances-by-slot 'friend-relation  
'second x))))

Of course there are ways to do this more efficiently, but I think this  
is the idea.

Ian

On Mar 16, 2008, at 11:06 PM, Marc wrote:

> Hello!
>
> I fear that this may be a rather stupid newbie question, but neither  
> from the documentation nor from the mailing list posts I got an idea  
> as to the best practice for the following type of searches. Let's  
> use an example akin to the one in the tutorial:
>
> (defpclass person ()
> ((name :accessor name :index t)
> (friends :accessor friends)) ;collection with names (say as strings)  
> of the friends of this person
> )
>
> Given a similar structure, what is the recommended way to handle  
> queries of the type "find all persons that X is a friend of?" (i.e.  
> queries on individual entries in slots that contain collections)?  
> What would be the best data structure for the friends slot, assuming  
> that the collection remains fairly stable over time (psets, lists or  
> other collections)?
>
> Thanks in advance for any hint!
>
> Best regards,
>
> Marc
>
>
>
>
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