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From Dave Merrin <dmer...@ipasystems.co.uk>
Subject Re: Select Query on Table with no Primary Key
Date Thu, 07 Jun 2007 15:44:21 GMT
Hi John,

as mentioned to Andrus I'm going with the setting primary keys on 
columns. I actually found some documentation for the database which said 
which columns make the rows unique. This keeps things relatively simple.

BTW. also tried some prefetching today....cool stuff. I got some code 
down from 8 seconds down to 3 seconds.

Cheers for the help... support is the reason I'm using Cayenne rather 
than Hibernate.

Dave

John wrote:
> Hello Dave,
>
>
> You said you use a view and you will NOT modify the data, then try this:
>
> Your View name : MyView.
>
> --------------------------------------------------------------------------------------------------------------------------

>
> create another view call MyViewWithPK
> -------------------------------------------------
>
>
> SELECT e.* , (SELECT COUNT(*) FROM MyView e2 WHERE e2.name <= e.name ) 
> AS rownumber
> FROM MyView e
>
> // remember that name must be included in the view MyView
> // remember that this technique is slow, but i suppose this is your 
> last problem :)
> ---------------------------------------------------------------------------------------------------------------------------

>
> let me know if this work :)
>
>
> Sako
>
> Signature IT-Consult Armainak Sarkis
> ----- Original Message ----- From: "Dave Merrin" 
> <dmerrin@ipasystems.co.uk>
> To: <user@cayenne.apache.org>
> Sent: Thursday, June 07, 2007 3:36 PM
> Subject: Re: Select Query on Table with no Primary Key
>
>
>> Hi Andrus,
>>
>> I was playing about with putting in my own EntityResolver. For now 
>> I'm going to take your approach and see how I get on.
>>
>> Cheers,
>>
>> Dave
>>
>> Andrus Adamchik wrote:
>>> To give an example - if you have an EMPLOYEE table, you can make an 
>>> assumption that it is highly unlikely that there are two people with 
>>> the same name, born on the same date, and working in the same 
>>> department. On that assumption you can mark these 4 columns as PK in 
>>> the Modeler: FIRST_NAME, LAST_NAME, DATE_OF_BIRTH, DEPARTMENT_ID.
>>>
>>> Works well with views or tables (updateable or read-only) on any DB.
>>>
>>> Andrus
>>>
>>>
>>> On Jun 7, 2007, at 1:29 PM, Andrus Adamchik wrote:
>>>
>>>> Tore is right - for Cayenne to handle an object (whether read-only 
>>>> on read/write), it needs to know which column or columns uniquely 
>>>> identify each row. Now... you can fake a PK in your model, even if 
>>>> there's none in the db - just select a really unique combination of 
>>>> columns, and mark those columns as the PK in the modeler. I've 
>>>> mapped tables with such "imaginary PK" a lot.
>>>>
>>>> If it is not possible (i.e. duplicate rows are expected to be 
>>>> fetched), you will have to use DataRows.
>>>>
>>>> Andrus
>>>>
>>>>
>>>> On Jun 7, 2007, at 1:04 PM, Tore Halset wrote:
>>>>
>>>>> On Jun 6, 2007, at 16:12 , Dave Merrin wrote:
>>>>>
>>>>>> I'm trying to run a SelectQuery on a table with no primary key. 
>>>>>> Unfortunately it's not working. Can anybody help? I have no 
>>>>>> control over the database so I can't add in primary keys.
>>>>>
>>>>> As you know the PK are essential not only to update a row, but to 
>>>>> make sure a single row maps to a single DataObject in your context.
>>>>>
>>>>> Some database engines do have a unique invisible column. If your 
>>>>> database does this, then perhaps you could map that column as your 
>>>>> primary key? What database engine are you using?
>>>>>
>>>>> Regards,
>>>>>  - Tore.
>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>
>


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