hibernate-devel

[Hibernate] Are toplevel collections / subcollections a Bad Thing?

[Gavin K. <Gav...@ex...>]

I'm (again) considering deprecating toplevel collections and
subcollections=20
in Hibernate 2.0. I was never completely happy with this feature after I

implemented it, but it had taken so much effort to get it to work that I
left it in.

The case against toplevel collections is:
* the relational model is broken - there is no foreign key constraint
from=20
collection element row to the owning row
* subcollections complicate code in SessionImpl _significantly_

The case for keeping them is:
* they let you persist a Java "collection of collections"
* they let you have a single table / collection mapping that can be used
by=20
different owning classes
* instances of a toplevel collection can be passed between different
owners=20
(even between owners of a different class, potentially) without needing
to=20
remove and recreate the collection rows
* they currently work and I havn't needed to touch the complicated code
for=20
a long time now

So: Do people USE this feature?? Have people really actually found uses=20
for this stuff, or is it actually just making Hibernate harder to
understand=20
for the first time? Even if it has been useful, is perhaps *still*
undesirable,=20
because of concerns about data integrity? Perhaps we simply shouldn't=20
be encouraging this kind of relational model.....

I need advice on this.

Gavin


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Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?

[Christian B. <chr...@bl...>]

On 18 Dec (20:49), Gavin King wrote:

> So: Do people USE this feature?? Have people really actually found uses 
> for this stuff, or is it actually just making Hibernate harder to
> understand 
> for the first time? Even if it has been useful, is perhaps *still*
> undesirable, 
> because of concerns about data integrity? Perhaps we simply shouldn't 
> be encouraging this kind of relational model.....

I tried once to use toplevel collections, because I had a "collection of
collections" and the collection could be referenced by two different
classes.

The mapping wasn't hard to understand, but the restrictions on the
load/delete/update killed them for me. 

Using a Hibernate Session is straightforward if you don't use toplevel
collections, the only thing to keep in mind is handling bidirectional
associations with setparent/setchild on both ends. 

Toplevel collections add a lot more overhead to this handling routines,
which is what kept me from using it. As soon as I realized that, I
stopped even thinking about using a toplevel collection and change my
models accordingly. I bet a lot of people are doing the same, the
benefits of toplevel collections would be nice, but dealing with them is
not.

My point is: For every aspect thats on your "plus" side for toplevel
collections, find a different way of implementing it without a toplevel
collection, then compare the effort for both methods and decide wether
this justifies an additional idiom (tl collections) in Hibernate or not.

Data Integrity: I'm not a DB newbie, so I noticed the missing foreign
key on first sight. But this is just the implementation of toplevel
collections, which could be changed if the idiom itself is worth it. 

-- 
Christian Bauer
tu...@in...

Bi-directional relationships (was Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?)

[Jim D. <jim...@po...>]

Sorry for taking a tangent here but after spending a day trying to
figure out what SessionImpl was doing and to work out why my
bi-directional associations didn't work I conceded defeat and moved to
a unidirectional model.

How do you 'setparent/setchild on both ends'? Is guidance on this in
some documentation that I missed?

Thanks,
jim

jim...@po...

On Wed, Dec 18, 2002 at 02:45:46PM +0100, Christian Bauer wrote:
> On 18 Dec (20:49), Gavin King wrote:
> Using a Hibernate Session is straightforward if you don't use toplevel
> collections, the only thing to keep in mind is handling bidirectional
> associations with setparent/setchild on both ends. 

Re: Bi-directional relationships (was Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?)

[Mark W. <mor...@SM...>]

Jim Downing wrote:

>Sorry for taking a tangent here but after spending a day trying to
>figure out what SessionImpl was doing and to work out why my
>bi-directional associations didn't work I conceded defeat and moved to
>a unidirectional model.
>
>How do you 'setparent/setchild on both ends'? Is guidance on this in
>some documentation that I missed?
>  
>

Check out http://www.xylax.net/hibernate/.  Tom has pulled together some 
pretty useful examples and they also have notes on bi-directional 
associations.

-Mark

Re: Bi-directional relationships (was Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?)

[Christian B. <chr...@bl...>]

On 19 Dec (00:57), Mark Woon wrote:

> >How do you 'setparent/setchild on both ends'? Is guidance on this in
> >some documentation that I missed?
> 
> Check out http://www.xylax.net/hibernate/.  Tom has pulled together some 
> pretty useful examples and they also have notes on bi-directional 
> associations.

It is also an item in the FAQ.

-- 
Christian Bauer
tu...@in...

Re: Bi-directional relationships (was Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?)

[Jim D. <jim...@po...>]

On Thu, Dec 19, 2002 at 10:02:10AM +0100, Christian Bauer wrote:
> On 19 Dec (00:57), Mark Woon wrote:
> > >How do you 'setparent/setchild on both ends'? Is guidance on this in
> > >some documentation that I missed?
> > 
> > Check out http://www.xylax.net/hibernate/.  Tom has pulled together some 
> > pretty useful examples and they also have notes on bi-directional 
> > associations.
> 
> It is also an item in the FAQ.

Thanks - I'd used the advance in Tom's manual to do the mapping, but I
was trying to maintain the relationship in my persistent objects
(rather than making the client do it, as in the FAQ), which was
probably the cause of my problems.

Thanks,

jim

Re: Bi-directional relationships (was Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?)

[Christian B. <chr...@bl...>]

On 19 Dec (09:20), Jim Downing wrote:

> Thanks - I'd used the advance in Tom's manual to do the mapping, but I
> was trying to maintain the relationship in my persistent objects
> (rather than making the client do it, as in the FAQ), which was
> probably the cause of my problems.

Thats exactly what I'm doing:

class Child {

    public setParent(Parent newParent) {

        // Remove myself from old Parent
        if (this.parent != null) {
            this.parent.getChildren().remove(this);
        }

        // Add to new Parent
        newParent.getChildren().add(this);

        // Set new Parent
        this.parent = newParent;
    }
}

This is similar to the Composite Pattern (or better, a part of it):

http://www.idg.net/english/crd_composite_948016.html

-- 
Christian Bauer
tu...@in...

Re: Bi-directional relationships (was Re: [Hibernate] Are toplevel collections / subcollections a Bad Thing?)

[Max R. A. <ma...@eo...>]

Should we implement this paradigm in the codegenerator ? (of course as an
option)

Maybe something like this:

<property name="parent" class="eg.Parent">
 <metaattribute name="CodeGen.genCompositeGetSet" value="true"/>
</property>

Where the codegen would generate something like the code below if
"CodeGen.genCompositeGetSet" is set for the property and/or relationship.

/max

----- Original Message -----
From: "Christian Bauer" <chr...@bl...>
To: <hib...@li...>
Sent: Thursday, December 19, 2002 10:31 AM
Subject: Re: Bi-directional relationships (was Re: [Hibernate] Are toplevel
collections / subcollections a Bad Thing?)


> On 19 Dec (09:20), Jim Downing wrote:
>
> > Thanks - I'd used the advance in Tom's manual to do the mapping, but I
> > was trying to maintain the relationship in my persistent objects
> > (rather than making the client do it, as in the FAQ), which was
> > probably the cause of my problems.
>
> Thats exactly what I'm doing:
>
> class Child {
>
>     public setParent(Parent newParent) {
>
>         // Remove myself from old Parent
>         if (this.parent != null) {
>             this.parent.getChildren().remove(this);
>         }
>
>         // Add to new Parent
>         newParent.getChildren().add(this);
>
>         // Set new Parent
>         this.parent = newParent;
>     }
> }
>
> This is similar to the Composite Pattern (or better, a part of it):
>
> http://www.idg.net/english/crd_composite_948016.html
>
> --
> Christian Bauer
> tu...@in...
>
>
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