QGIS aggregate expression to filter selected features in 2nd layer

Question

I´m working with two layers in QGIS:

1. First one with point geometries representing water quality sampling places (LAYER A).

2. The second is a spreadsheet (no geometries) that contains pollution values at different times (LAYER B).

Both layers linked with a relate (one to many) based on the station ID. Relate works as the LAYER B data are preented in the info form when clicking LAYER A features.

The aim is to get time series graphs using Data Plotly plugin, but apparently it is not designed to work with relates.

I´ve come to the conclussion that the only way to get the plots is to set LAYER B in the plugin and then limit the feature subset to the records that match the STATION_ID of the selected feature in LAYER A. I´ve tried this with an expression of the sort

STATION_ID = 
aggregate(layer,aggregate,expression
[,filter][,concatenator])

where layer is LAYER A, aggregate is 'concatenate', and expression would be the field contaning the STATION_ID

but cannot guess what the expression of the filter should be so that only the data of the selected feature is presented in the plot. Also i´m aware that in case that more than one features are selected, the concatenate wouldn`t work.

Any idea how to solve this? Another way to get the selected STATION_ID from LAYER A?

Answer 1

Not sure if I understand your question correctly, but here is how you can get selected features of another layer using aggregate() expression as asked in your question's title:

aggregate('LayerB','array_agg',"id",is_selected('LayerB',$currentfeature))

Of course you can also use other stuff than 'array_agg' and other fields/features/geometries than "id". Also you could get the not selected features, by simply using not in front of is_selected():

aggregate('LayerB','array_agg',"id",not is_selected('LayerB',$currentfeature))

The trick is to pass two arguments to the is_selected() function in the filter argument of aggregate(): the layername and the feature.

If the function is called with both a layer and a feature, it will return true if the specified feature from the specified layer is selected.