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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: Can we have an entity with no @Id?
Date Wed, 20 May 2009 17:49:09 GMT
Hi,

Tables without primary keys is one of the features of JDO that was not  
adopted by JPA.

Maybe you should look at JDO implementations.

Craig

On May 20, 2009, at 8:12 AM, is_maximum wrote:

>
> Hello
>
> To Andrei I want to say that because it is simple to create an  
> object and
> send it to be persisted however this could be a good idea.
>
> And to Kevin, if in secondary table we have only a foreign key to  
> distinct
> records that would be enough because the id for the secondary table   
> is not
> used anywhere. All we need from these two tables is a report that  
> tells us
> what kind of events (secondary table) have been occurred for a  
> specific
> operation in specific time (master table) and order is not important  
> since
> the time of event will order those records in right way. So what is  
> this ID
> good for? In case of existence of this ID we have to either create  
> an Oracle
> sequence (which is a bottleneck in database because it takes up cache
> particularly in a clustered environment) or selecting the maximum ID  
> (which
> is required to scan all the records) or create a sequence table  
> managed by
> ORM (That will end up with a select statement and an update  
> following that)
> or create that id manually (that it is almost impossible)
>
>
> Kevin Sutter wrote:
>>
>> Hi is_maximum,
>> I'm still a little confused by your scenario.  Following your  
>> described
>> scenario...  Your master table would have an Id field, but your  
>> secondary
>> table would not have an explicit Id field.  The foreign key from your
>> master
>> to secondary would just be some arbitrary column from the secondary  
>> table?
>> Do I have that right?  And, why would removing an Id field help with
>> performance?  You mention to get rid of its sequence, but there's no
>> requirement to define an Id field with a sequence.
>>
>> Even though I'm still a little confused by your scenario, there are a
>> couple
>> of items to be aware of from an OpenJPA perspective.  The JPA spec
>> requires
>> an Id field, but OpenJPA does not require one.  Well, not exactly.
>> Instead
>> of declaring an explicit Id field, you could instead declare an Id  
>> via the
>> @DataStoreId annotation [1].  This hides the Id field from your  
>> Entity
>> definition, but under the covers we still use an implicit Id field in
>> order
>> to insert and find records.
>>
>> Another possibility is coming with the updated JPA 2 specification  
>> and the
>> use of derived identities.  I'm probably stretching this one a bit,  
>> but
>> this
>> support would allow you to derive an identity for an Entity based  
>> on the
>> identity of a "parent" Entity.  This is normally used when the  
>> dependent
>> Entity is the owner of a many-to-one or one-to-one relationship to  
>> the
>> parent entity.  Here again, OpenJPA provides a similar  
>> functionality with
>> their Entities as Identity fields.
>>
>> So, bottom line is that some type of Identity is required for proper
>> Entity
>> definition and usage.  But, OpenJPA (and eventually the JPA 2 spec)
>> provides
>> for some methods to get around the explicit definition of an Id  
>> field.
>>
>> Hope this helps,
>> Kevin
>>
>> [1]
>> http://openjpa.apache.org/builds/latest/docs/manual/manual.html#ref_guide_pc_oid_datastore
>> [2]
>> http://openjpa.apache.org/builds/latest/docs/manual/manual.html#ref_guide_pc_oid_entitypk
>>
>> On Wed, May 20, 2009 at 7:14 AM, is_maximum <mnrz57@gmail.com> wrote:
>>
>>>
>>> Hi
>>> We have some tables in which id is not important and actually it is
>>> useless.
>>> For example we have two logging tables, one is master and the  
>>> other is
>>> keeping details. The only foreign key from the master table is  
>>> enough and
>>> the second table has no relationship with other tables so if we  
>>> remove
>>> its
>>> ID we can get rid of its sequence and this will be great in terms of
>>> performance since this table is considered to keep lots of records  
>>> and
>>> its
>>> purpose is for preserving events took place in the system
>>>
>>> Now my question is how to remove its @Id from the entity since the
>>> OpenJPA
>>> complains if the entity has no field marked as id
>>>
>>> Thanks
>>> --
>>> View this message in context:
>>> http://n2.nabble.com/Can-we-have-an-entity-with-no-%40Id--tp2945752p2945752.html
>>> Sent from the OpenJPA Users mailing list archive at Nabble.com.
>>>
>>>
>>
>>
>
>
> -----
> --
> Regards
> Mohammad
> http://pixelshot.wordpress.com Pixelshot
> -- 
> View this message in context: http://n2.nabble.com/Can-we-have-an-entity-with-no-%40Id--tp2945752p2946693.html
> Sent from the OpenJPA Users mailing list archive at Nabble.com.
>

Craig L Russell
Architect, Sun Java Enterprise System http://db.apache.org/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


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