users@jpa-spec.java.net

[jpa-spec users] [jsr338-experts] Re: Proposal for EntityGraphs, fetch plans, etc...

From: Emmanuel Bernard <ebernard_at_redhat.com>
Date: Mon, 17 Dec 2012 18:09:14 +0100

OK I get it now.

To be honest I am quite uncomfortable with such a feature. Does everyone
really think that this feature is a must have?

I imagine that the use case you have in mind is to pass this copied
object graph to another serialization or traversal framework that would
otherwise traverse the JPA guards like there is no tomorrow.

Our experience with Bean Validation has shown that handling guards is
extremely easy and that the necessary contract to make a traversal /
serialization framework aware of these is quite simple
https://github.com/beanvalidation/beanvalidation-api/blob/master/src/main/java/javax/validation/TraversableResolver.java

The solution proposed here (the copy) forces to copy an object graph
which leads to memory duplication just to get the data serialized in
another form. Plus the user *must* use the same graph definition for
both `copy` and `merge` or else data loss are going to happen. I find
the programming model quite error prone and dangerous.

Emmanuel

On Mon 2012-12-17 9:40, Gordon Yorke wrote:
> Hello Emmanuel,
> The goal of a copy graph is to produce a simple subgraph copy of
> the entity graph for environments where enhancement/proxies may not
> be available or where these "guards" can not be retained when then
> graph is serialized. The developer has a template in the form of
> the entity graph which provides the details of which attributes have
> legitimate values and which ones should be ignored and this template
> is highly recommended for the merge. There is no requirement for
> "guards" embedded within the copies. In client/server environments
> where "guards" would be available there is no need for copy() at all
> as the same functionality is achieved serializing entities loaded
> with a fetch graph.
>
> --Gordon
> On 17/12/2012 5:25 AM, Emmanuel Bernard wrote:
> >>>## Use cases
> >>>
> >>>I would really like to get use cases associated to each of the new
> >>>features proposed here.
> >>>
> >>>In particular:
> >>>- it's unclear to me why you really need loadgraph and fetchgraph.
> >>>- why is merge( fetchgraph ) required when you can achieve the same with
> >>> - find( fetchgraph )
> >>> - detach / serialize or whatever
> >>> - merge() regular method
> >>merge(EntityGraph) applies when the user is using copy(EntityGraph)
> >>to detach an entity tree for serialization. If an attribute is not
> >>present in the copy then it must not be merged. So, your example
> >>becomes:
> >> - find (EntityGraph eg1)
> >> - copy(EntityGraph eg1)
> >> - serialize out/in
> >> - merge(EntityGraph eg1)
> >Sure but the graph returned by copy does contain guards on uninitialized
> >properties and associations right? Ie when a non copied property /
> >association is traversed, some exception is raised.
> >
> >If that's not the case then what are you returning? null or the
> >primitive default values? I would be strongly against that.
> >
> >So assuming the engine has those guards on uninitialized properties and
> >associations, then the merge with an EntityGraph is not necessary as
> >this information can be discovered (like we do for a regular merge).
> >
> >Emmanuel
>