[elephant-devel] storing instances of non-persistent class

[Frank Schorr]

I was surprised to find that also objects which are 
not instances of a persistent-metaclass are handled and 
stored by elephant without complaints 
if they are slot values of a persistent class. 

In my code shown below, I expected to receive an undefined method error, 
similar to what is suggested by page 41 of 

http://www.franz.com/support/documentation/8.0/acache/doc/acache.pdf

which requires that an encode-object method must be provided to indicate 
how a non-persitent object can be saved. Obviously, though this is not 
mentionned in the documentation, elephant can find this 
out by its own.

Question 1: Can I rely on this, are there limitations ?


The following commands work:

(defclass nonpers-class ()
  ((slot-a :accessor slot-a :initarg :slot-a)
   (slot-b :accessor slot-b :initarg :slot-b)))

(defmethod calc-key ((obj nonpers-class))
  (slot-a obj))

(defclass pers-class ()
  ((myslot :accessor myslot)
   (myslot-key :index t))
  (:metaclass persistent-metaclass))

(defmethod (setf myslot) :after (new-value (obj pers-class))
  (setf (slot-value obj 'myslot-key) (calc-key new-value)))

(setf inst-1 (make-instance 'nonpers-class :slot-a "aaa" :slot-b "111"))
(setf inst-2 (make-instance 'nonpers-class :slot-a "bbb" :slot-b "222"))
(setf inst-3 (make-instance 'nonpers-class :slot-a "ccc" :slot-b "333"))

(setf pers-1 (make-instance 'pers-class))
(setf (myslot pers-1) inst-1)

(setf pers-2 (make-instance 'pers-class))
(setf (myslot pers-2) inst-2)

(setf pers-3 (make-instance 'pers-class))
(setf (myslot pers-3) inst-3)

(get-instances-by-value 'pers-class
                        'myslot-key 
                        (calc-key (make-instance 'nonpers-class :slot-a "bbb" :slot-b "222")))

This means a key is calculated for objects of a non-persistent class, 
and this key is used to retrieve such objects.

Question 2: Do you think such approach is ok or way too complicated ?

I would be happy to receive comments on the above.

Best regards,
Frank Schorr

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