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From Robert Zeigler <robert.zeig...@roxanemy.com>
Subject Re: Interfaces for domain objects?
Date Wed, 13 May 2009 17:38:13 GMT
It's certainly true that it's possible with JPA, but, at least when  
using Hibernate as the JPA provider, if you don't explicitly fiddle  
with the mappings, you get an intermediate table by default; at least,  
I've always had to add extra mapping information to achieve an  
intermediate-tableless schema.

Robert

On May 13, 2009, at 5/1312:31 PM , Mike Kienenberger wrote:

> It's certainly possible to develop templates that provide entity
> interfaces.  I've done so in past projects.
>
> And if you mess around with the generic signatures enough, you can
> probably solve those problems as well.
>
> While I won't recommend JPA (since I am now having to use it instead
> of Cayenne), it's also not a problem to define 1:n relationships
> without intermediate join tables.   We have many one-to-many and
> many-to-one relationships defined in JPA, and none of them have
> intermediate join tables.  In fact, you can have a one-to-many
> relationship defined in a single object, so it's not a JPA issue.   I
> still recommend using Cayenne instead of JPA, though.
>
> object:
>   - column: ID
>   - column: PARENT_ID
>   - relationship: parentObject
>   - relationship: childObjectList
>
>
>
> On Tue, May 12, 2009 at 4:40 AM, Jeremias Maerki <dev@jeremias-maerki.ch 
> > wrote:
>> Hi there,
>>
>> after years of not having to deal with databases, I'm back to that  
>> topic.
>> So I'm still figuring out the best way for myself to do ORM. My first
>> two attempts were to use JCR (Jackrabbit) and JPA (OpenJPA). JCR  
>> worked
>> fine although I dropped it because I can easily have >100'000  
>> children
>> on one node which will result in performance problems or require
>> work-arounds that I don't particularly like. JPA then just went  
>> against
>> my views of how databases should be modeled: an intermediate table  
>> for a
>> simple 1:n relation. Not with me. I couldn't get the whole thing to  
>> work
>> without the intermediate table. Then I stumbled upon Cayenne (I'm  
>> using
>> the Trunk) and immediately found myself at home.
>>
>> Now, my problem: I've been trying to make an abstraction of my data
>> logic using interfaces, i.e. define an interface for each domain  
>> object
>> and logic methods grouped in an interface per topic. That works  
>> pretty
>> well for the JCR and JPA implementations. I wrote the test code  
>> once and
>> can reuse it for all implementations. With Cayenne, this gets a bit
>> difficult. I can always implement the domain interfaces in the domain
>> classes but when it comes to nested collections, this becomes a  
>> problem:
>>
>> public interface Document extends Serializable {
>>
>> [..]
>>
>>    Collection<Representation> getRepresentations();
>>
>> }
>>
>> public interface Representation extends Serializable {
>> [..]
>> }
>>
>> Generated domain class:
>>
>> public class DocumentImpl extends _DocumentImpl implements Document {
>> [..]
>> }
>>
>> public abstract class _DocumentImpl extends CayenneDataObject {
>> [..]
>>    @SuppressWarnings("unchecked")
>>    public List<RepresentationImpl> getRepresentations() {
>>        return  
>> (List<RepresentationImpl>)readProperty("representations");
>>    }
>> }
>>
>> List<RepresentationImpl> is not compatible with the  
>> List<Representation>
>> in the interface.
>>
>> So, I'm wondering if there's a better way to do this that avoids this
>> problem, preferably without removing generics.
>>
>> I was thinking that adding the interface in the object model could
>> optionally be specified and the code generator could, for example,  
>> do an
>> "implements Document" for _DocumentImpl and return  
>> List<Representation>.
>> Of course, that would require some casting to DataObject in
>> addToManyTarget() for example. That's probably undesired and would  
>> probably
>> require quite a bit of work. But maybe there's a better way. I could
>> probably proxy/adapt the whole stuff but that's probably bad
>> performance- and effort-wise. Maybe I simply have to give up my  
>> puristic
>> data access abstraction. But I wanted to ask around first.
>>
>> ---
>>
>> While I'm at it: Can I ask why DeleteQuery has been deprecated? I'd
>> rather use that instead of EJBQLQuery. Not having to concatenate  
>> strings
>> to build queries is one of the features that makes Cayenne appeal  
>> to me
>> so much. I've had stuff like that back in my Delphi times and I liked
>> that very much.
>>
>>
>> Thanks a lot for any ideas and thoughts,
>> Jeremias Maerki
>>
>>


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