In the most general case adding support for a new kind of expression requires
the following:
1. Support in ExpressionFactory and Property classes (though see comment below
on CollectionProperty)
2. Support in Expression parser (so that the expression can be created from
String)
3. Support in SelectTranslator (so that the expression can be converted into
SQL).
In your case a minimal implementation can be quite simple: implement #1 to work
as outer join example ("a.paintingArray+ = null"). So existing #2 and #3 will
be reused. Perhaps we create a Property subclass for to-many relationships
(CollectionProperty or something) that will have that extra method for empty.
Then you will also need to tweak cgen templates so that CollectionProperty is
included in generated classes.
Next step may be adding EJBQL syntax to Expression parser (#2): path IS [NOT]
EMPTY
And finally we may decide to generate SQL different from OUTER JOIN (EXISTS,
etc.). Only here this expression becomes truly "new". If you get to this step,
let's discuss it separately on this list. We will need to discuss the behavior
and SQL we should generate. Also Alex Kolonitsky is now working on a new
SelectTranslator to include aggregates, etc., so this step will likely overlap
with his work.
Andrus
> On Jul 24, 2015, at 7:56 PM, Hugi Thordarson <[email protected]> wrote:
>
> Indeed :). I’ve been looking at the Expression classes for a while now and
> they look a little daunting. Any recommendations on where to start? Is there
> a developer guide or an annotated example implementation somewhere?
>
> - hugi
>
>
>
>> On 24. júl. 2015, at 15:59, Andrus Adamchik <[email protected]> wrote:
>>
>> Classic open source - "scratch your own itch" :)
>>
>>
>>> On Jul 24, 2015, at 6:54 PM, Hugi Thordarson <[email protected]> wrote:
>>>
>>> Thanks Andrus, we’ll revert to EJBQL for now.
>>>
>>> Looks like an excellent opportunity to try and implement my first
>>> Expression class :)
>>>
>>> - hugi
>>>
>>>
>>>
>>>> On 24. júl. 2015, at 15:36, Andrus Adamchik <[email protected]> wrote:
>>>>
>>>> We should add it. For now OUTER JOIN should do the trick:
>>>>
>>>> "a.paintingArray+ = null"
>>>>
>>>> Also EJBQL syntax supports "IS [NOT] EMPTY" :
>>>>
>>>> "SELECT a FROM Artist a WHERE a.paintingArray IS EMPTY"
>>>>
>>>> Andrus
>>>>
>>>>> On Jul 24, 2015, at 5:51 PM, Hugi Thordarson <[email protected]> wrote:
>>>>>
>>>>> Hi all.
>>>>> Does Cayenne have an Expression class that allows searching for objects
>>>>> with an empty to-many relationships? If not, any pointers towards a
>>>>> sensible approach?
>>>>>
>>>>> Cheers,
>>>>> - hugi
>>>>>
>>>>> // Hugi Thordarson
>>>>> // http://www.loftfar.is/ <http://www.loftfar.is/>
>>>>> // s. 895-6688
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>
