[elephant-devel] Storing objects in the root when don't care about the key / add same key to root more than once

[Joubert Nel]

On Mon, 2007-05-28 at 08:51, Ian Eslick wrote:
> A few clarifications:
> 
> Standard persistent classes are not saved by default.  To get access  

Ian, when are objects that are instanced from persistent classes saved? 
I simply call (make-instance) on the following class definition:

(defclass risk ()
  ((title :accessor title :initarg :title :initform ""))
  (:metaclass elephant:persistent-metaclass)
  (:index t))

the resulting object is immediately saved/persisted by Elephant... 


> to a class via the map-class operator the class or a slot of the  
> class must be declared as :indexed.  If a slot is indexed, then that  
> value can be used as a key for the object via the get-instances-by- 
> value interface.  Moreover, you can have duplicate slot values and  
> get-instances-by-value will return all the objects having that slot  
> value.

Understood - this is what I got from the documentation as well.

> 
> Think of the root more like a hash table (single key-value pair)  
> instead of a set or collection.  The simple get-value interface would  
> be more complex if it could return multiple values.

My impression until I saw how persistent objects behave was that
everything you want to store via Elephant had to be "rooted". 

> 
> It is poor coding practice to use system-assigned OIDs.  In future  
> versions of elephant OIDs may change over time due to GC, auto  
> recovery of oid space, etc.  It's sort of like using the physical  
> address of an object in a C++ program.  There is a legitimate way to  
> use OIDs outside of elephant, but we haven't yet defined an interface  
> for users to do so (for example you might want to use OIDs to create  
> many-to-many maps or efficient persistent sets)

Thanks for this insight. Many-to-many maps and sets are exactly what I'm
trying to achieve.
I found a newly released UUID package for Lisp -
http://www.cl-user.net/asp/libs/uuid 
I'm going to explore using this instead of OIDs.

Joubert