Re: Expression to look for objects with an empty to-many relationship?
Andrus/Alex, If the AggregateUtils I put together is helpful, feel free to use it. Thanks, mrg On Mon, Jul 27, 2015 at 4:49 AM, Andrus Adamchik wrote: > 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. >
Re: Expression to look for objects with an empty to-many relationship?
Thanks a lot Andrus! I’m going to play around a little with this and see if I
can’t make something useful of it :)
Cheers,
- hugi
> On 27. júl. 2015, at 08:49, Andrus Adamchik wrote:
>
> 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 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 wrote:
>>>
>>> Classic open source - "scratch your own itch" :)
>>>
>>>
On Jul 24, 2015, at 6:54 PM, Hugi Thordarson 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
> 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 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/
Re: Expression to look for objects with an empty to-many relationship?
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 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 wrote:
>>
>> Classic open source - "scratch your own itch" :)
>>
>>
>>> On Jul 24, 2015, at 6:54 PM, Hugi Thordarson 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 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 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/
