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From Wernke zur Borg <wernke.zur.b...@esa.int>
Subject Re: Syntax error and question about JOIN
Date Tue, 28 Jun 2011 07:52:14 GMT

Thanks for your answers. I would like to stick to version 3.1, so could
somebody give me a hint as to where to modify the JOIN syntax in a
custom DB adapter? I looked at all the existing DB adapters and I guess
I would subclass JdbcAdapter but I could not find a suitable method to
override.

Thanks, Wernke


Am 21.06.2011 15:41, schrieb Andrus Adamchik:
> Past versions of Cayenne used joins in WHERE clause, that were likely compatible with
MS Access. So older Cayenne 2.0.x should probably work. 
>
> Also writing a custom Access DbAdapter that adds INNER should be possible. 
>
> Andrus
>
> On Jun 21, 2011, at 11:36 AM, Aristedes Maniatis wrote:
>> On 21/06/11 6:02 PM, Wernke zur Borg wrote:
>>> Hi,
>>>
>>> I am having a few problems with queries on an MS Access database when I
>>> use object relationships to access related tables. I am using Cayenne
>>> 3.1 M2.
>>>
>>> My case is very simple. Let's say I have two tables A and B where A has
>>> a column col containing a primary key of B. Given an object a from table
>>> A with colValue, I want to access the related record of B using a.getB().
>>>
>>> Cayenne creates a select query that joins A and B like this:
>>>
>>> SELECT DISTINCT t0.columns... FROM B t0 *JOIN* A t1 ON (t0.pk = t1.col)
>>> WHERE t1.pk = ? [bind: 1->  a.pkValue]
>>>
>>> The first thing is that this produces a syntax error as the simple *JOIN
>>> *keyword is not valid in MS Access. They want an explicit *INNER JOIN*.
>>
>> I've never heard of anyone using MS Access before as an SQL engine, so this might
be one reason why. I would run, not walk, to another db if you can at all manage that within
your environment.
>>
>>
>>> But secondly I wonder why Cayenne needs a JOIN in this case anyway,
>>> since it is of no use at all. Why does it not use something like:
>>>
>>> SELECT t0.columns... FROM B t0 WHERE t0.pk = ? [bind 1->  a.colValue]
>>>
>> I guess since any SQL engine will optimise that out, the fact that Cayenne does it
in the most generic way usually doesn't matter. I haven't seen the code in question specifically,
but Cayenne performs JOINs very often and has a bunch of code which knows how to do that (multiple
keys, qualifiers, inheritance, etc) and that code is just being reused here.
>>
>>
>>> My workaround is to manually create a SELECT query to access B without
>>> joining it A:
>>>
>>> Expression e = ExpressionFactory.matchExp(B.primaryKeyProperty, A.colValue);
>>> SelectQuery q = new SelectQuery(B.class, e);
>>>
>>> This works but of course it would be nicer to use the getter function
>>> A.getB().
>>>
>>> Is there a possibility to either avoid the JOIN or tell Cayenne to use
>>> INNER JOIN instead of JOIN ?
>> Yes, you can write specific db adapters for databases of your choice. You'll need
to get your hands into Cayenne itself, but the classes in question aren't too hard to understand.
Let us know if you need more pointers to do that. I am guessing that none of the development
team will be easily able to test against Access, and we don't currently run any unit tests
against Access. But if you can help make it work, then so much the better.
>>
>> Cheers
>> Ari
>>
>> -- 
>> -------------------------->
>> Aristedes Maniatis
>> GPG fingerprint CBFB 84B4 738D 4E87 5E5C  5EFA EF6A 7D2E 3E49 102A
>>


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