[DISCUSS] Convenient PC method introductions

[Matthew Adams <ma...@xcalia.com>]

This was discussed on the Feb 23 JDO conf call.  Please review and
discuss.

Overview:
Grails is a promising new framework for lightweight, Rails-style
development.  It uses GORM, which introduces several nice features to
persistent objects, but it is hard-coded against Hibernate and won't
currently run outside of the Grails application framework.

Motivation:
* Frameworks like GORM are currently specific to Hibernate only, and
they can't (to my knowledge) be extracted from Grails
* Spring provides some conveniences via JDOTemplate that would be nice
in general.

Spring has several convenient methods offered via JDOTemplate that can
be offered in an intuitive way in a manner similar to GORM.

With the introduction of JDO 2.1's PMF.getPersistenceManagerProxy(),
there are new possibilites to introduce to persistent classes convenient
static & instance methods without any work on the developer's part.

Proposed solution:
* Introduce convenient static & instance methods on persistent classes
* Identify a way to make PMF available across application
* Leverage pmf.getPersistenceManagerProxy() in introduced methods

Details:

There are some catchy (the author would say "hilarious") names for
Groovy persistence integration, as that was my original thought:

Gentities
Grentities
Perstroovy
Gersistence
Groovistence
Proovy
Proovy Gersistence
GDO:  Groovy Data Objects == Groovy + JDO
GPA:  Groovy Persistence API == Groovy + JPA
["GORM" is currently "Grails ORM" == Groovy + Hibernate within Grails
(should have been Ghibe, Ghibernate, Gribe or Gribernate to be more
accurate)]

Introducing via AOP is another possibility, but they're not as good as
the stuff above:

Paop (pronouned "pow-p":  Persistence with AOP)
Aspersistence (kind of like "aspirin")
Aspectistance
Aspentities
AOE (pronouned "ow":  Aspect-oriented Entities)
Jadaop (pronouned "jah dow-p": JDO & AOP)

Enough with the catchy names.

* Need some way to make PMF available to application, like Grails does
with GORM
    * InheritableThreadLocal holding a map of PMF by name
(Map<String,PersistenceManagerFactory>)?
    * PMF Proxy?  JDOHelper.getPersistenceManagerFactoryProxy(???) ?
    * Craig says this is not very hard -- placeholder for this
discussion is "JDO.getApplicationPMF()"

* Leverage pmf.getPersistenceManagerProxy() in order to enable:

Static methods that can be introduced:

// Foo is a persistence-capable class, btw.
// Groovy syntax here, btw -- not sure if it's totally correct

/** static method to return an instance of the class gotten by id */
get { id ->
 
JDO.getApplicationPMF().getPersistenceManagerProxy().getObjectById(getCl
ass(), id)
}
/** static method to count the number of instances of the class */
count() {
    PersistenceManager pm =
JDO.getApplicationPMF().getPersistenceManagerProxy()
    Query q = pm.newQuery(getClass(), "count(this)")
    q.execute()
}
...

instance method introduction
getId() { JDOHelper.getObjectId(this) }
save() {
JDO.getApplicationPMF().getPersistenceManagerProxy().makePersistent(this
) }
delete() {
JDO.getApplicationPMF().getPersistenceManagerProxy().deletePersistent(th
is) }
detachCopy() {
JDO.getApplicationPMF().getPersistenceManagerProxy().detachCopy(this) }
...

We can support JPA & JDO pretty easily if we can get either an EMF or
PMF like Something.getPersistenceFactory() that returns Object:

count() {
    Object factory = Something.getPersistenceFactory(); // returns
either EMF or PMF
    if (factory instanceof PersistenceManagerFactory) {
        PersistenceManager pm = ((PersistenceManagerFactory)
factory).getPersistenceManagerProxy()
        Query q = pm.newQuery(getClass(), "count(this)")
        return q.execute()
    }
    else if (factory instanceof EntityManagerFactory) {
        EntityManager em = ((EntityManagerFactory)
factory).getEntityManager() // the "right" one?
        Query q = em.createQuery("SELECT count(x) from " +
this.getClass().getName() + " x")
        return q.getSingleResult()
    }
    else {
        throw new YouAreTooAdvancedForMeException(...);
    }
} 
 
Matthew T. Adams
Senior Consultant & Product Marketing Director
Xcalia, Inc.
matthew.adams@xcalia.com
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+1 253 732 1051 Mobile
+1 815 331 0952 Fax
http://www.xcalia.com <http://www.xcalia.com/> 
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