[elephant-devel] Newbee questions

[Robert L. Read]

Dear William,
    Welcome.

    Are you using 0.5 or the latest version from CVS (which has not
quite been formally released yet)?  
I think this changes slightly based on which version you are using.

    Under the 0.5:

    I think the "Tutorial" on the section Persistent Classes 
http://common-lisp.net/project/elephant/doc/Persistent-
Classes.html#Persistent-Classes
shows how to do what you want; I'm pretty sure that it is accurate, but
haven't actually done it.
It shows placing the whole object into the root; I honestly would have
to go back to 0.5 to 
say whether this is really necessary or not.  You can try my code
snippet below under 0.5 
and see if it works.  (We have been working on what will be 0.6 for
quite a while.)

    Under the most recent CVS version:

    Yes, each time you make-instance a class with a metaclass
of :persistent-metaclass, the instance
is placed in the database with a unique OID.  If you store the OID (for
example in the root), you
can can then close the store-controller, exit lisp, restart LISP, reopen
the store-controller, and 
then either do (make-instance  'my-persistent-class :from-oid OID), or,
if you have indexed a slot
of enables class indexing on the class, you could you (get-instance-by-
class 'my-persistent-class)

ELE-TESTS> (defclass my-persistent-class ()
        ((slot1 :accessor slot1)
         (slot2 :accessor slot2))
        (:metaclass persistent-metaclass))
     
#<PERSISTENT-METACLASS MY-PERSISTENT-CLASS>
ELE-TESTS> (setq x (make-instance 'my-persistent-class))
; loading system definition from /usr/local/share/lisp/ele-bdb.asd into
; #<PACKAGE "ASDF1474">
; registering #<SYSTEM ELE-BDB {AA32BE1}> as ELE-BDB
STYLE-WARNING: implicitly creating new generic function BUILD-BTREE-
INDEX
#<SLEEPYCAT::BDB-STORE-CONTROLLER {B6C1941}>
ELE-TESTS> (setq x (make-instance 'my-persistent-class))
#<MY-PERSISTENT-CLASS {B8BA3C1}>
ELE-TESTS> (elephant::oid x)
14700
ELE-TESTS> (close-store)
NIL
ELE-TESTS> (open-store *default-spec*)
#<SLEEPYCAT::BDB-STORE-CONTROLLER {A97B6B1}>
ELE-TESTS> (make-instance 'my-persistent-class :from-oid 14700)
#<MY-PERSISTENT-CLASS {A9FEE69}>
ELE-TESTS> 

In the file classindex.lisp, you can find:

(defgeneric get-instances-by-class (persistent-metaclass))
(defgeneric get-instances-by-value (persistent-metaclass slot-name
value))
(defgeneric get-instances-by-range (persistent-metaclass slot-name start
end))

which are very useful for this, and may mean you don't have to store the
OID in the root if 
you "enable-class-indexing" on the class.


    
    

On Thu, 2006-03-02 at 17:11 -0500, William Halliburton wrote:

> 
> Starting to use elephant with bdb backend. 
> 
> Please tell me if the following explanation is correct.
> 
> Each time I make-instance a class with a metaclass of :persistent-
> metaclass the class is placed into the database with a unique OID. I
> can retrieve this back with (make-instance 'class :with-oid OID). Now
> this object in not in the root. Must I place it into the root? How do
> I remove old objects?
> 
> Thank you,
> William Halliburton
> 
> _______________________________________________
> elephant-devel site list
> elephant-devel at common-lisp.net
> http://common-lisp.net/mailman/listinfo/elephant-devel
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