[QGIS-Developer] QGIS expression to get internal ID of a feature in a related layer

[Andreas Neumann]

Hi Stefan,

I got it work with "aggregate()" not with "relation_aggregate()"

Not sure if relation_aggregate() works in both directions?

I also got a warning "The generated relations for a polymorphic relation 
cannot be used" ...

I have inheritance in my data model - maybe that is the cause that 
relation_aggregate() doesn't work, while aggregate works.

------------

Here is what works for me:

with_variable(
   'vereinbarung',
   get_feature(
     'Vereinbarung',
     't_id',
     vereinbarung
   ),
   aggregate(
     layer:='Vereinbarung',
     aggregate:='array_agg',
     expression:=@id,
     filter:=t_id = attribute(@vereinbarung,'t_id')
     )[0]
)

'Vereinbarung' is a geometry layer in my project that is related to a 
no-geometry table from which I want to trigger an atlas rendering ...

It is a pity that a "feature" doesn't contain it's internal ID in the 
attributes ... it would make it much easier! No silly workarounds with 
aggregates to get access to an internal id ...

Thanks a lot for the idea with the aggregates!

Andreas

On 2022-11-29 16:54, Stefan Giese (WhereGroup) via QGIS-Developer wrote:

> Hi Andreas,
> 
> in case of a relation 1:many you can use the _relation_aggregate_ 
> function, something like:
> 
> _ relation_aggregate('test_n_318_fk_test1_7993_id','concatenate',__ 
> to_string(@id),',')_
> 
> where the first parameter is the ID of the relation.
> 
> Hope this helps...
> Stefan
> 
> Am 29.11.2022 um 15:05 schrieb Andreas Neumann via QGIS-Developer:
> 
>> Hi,
>> 
>> I am looking for a way to find out the internal id of a related layer 
>> with a QGIS expression.
>> 
>> @id and $id only work directly on the feature of the source layer, but 
>> not on related layers.
>> 
>> attributes(
>> get_feature(
>> 'Other Related Layer',
>> 't_id',
>> fkey_value
>> )
>> )
>> 
>> doesn't contain the internal id of the other layer ...
>> 
>> Any ideas how to get access to internal ids of other layers?
>> 
>> Thank you and greetings,
>> 
>> Andreas
>> 
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> --
> Mit freundlichen Grüßen
> Stefan Giese
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