Listing IDs of all selected features with expression in QGIS

Question

I would like to get an array with IDs of all selected features of layer 'LayerB' (field-name: "id_of_layerB") as a default value in a QGIS Attribute form of 'LayerA'.

I tried something like:

aggregate(layer:='LayerB',
          aggregate:='array_agg',
          expression:= "id_of_layerB",
          is_selected('LayerB', ???)
          )

but I have no idea how to realize this (especially what to put in the ???-part of the expression part of the code.

I think, the is_selected()-function checks agains just one feature, but is there a way to get ALL (one or more) selected features of 'LayerB'?

Answer 1

Try to adjust your expression like this:

string_to_array(
    aggregate(@layer,
              aggregate:='concatenate',
              expression:=to_string("id"),
              filter:=is_selected(),
              concatenator:=','),
    delimiter:=',')

Answer 2

Used the pyqgis-code from @Taras, renamed some arguments and added some flexibility (layerName as argument) and a check against unique layername and added a helptext :

from qgis.core import *
from qgis.gui import *


@qgsfunction(args='auto', group='Custom')

def selected_content(layerName, field, feature, parent):
    """
    Returns a array of field-contents of selected features<br><br>
    Returns <br><br><span class=\"argument\">string </span>Array of field-contents
    <br>
    <h4>Syntax</h4><br>
    <code><span class=\"functionname\">selected_features</span> <span class=\"argument\">(string, string)</span></code><br>
    
    <h4>Arguments</h4><br>    
    <span class=\"argument\">string </span>UNIQUE layer name <br>
    <span class=\"argument\">integer </span>field name <br>
    <h4>Example</h4><br>    
    <ul> 
      <li><code>selected_content('myLayerName','myFieldName')</code> &rarr; <code>'['a','b']'</code>
    </ul>

    """
    layer = layers = QgsProject.instance().mapLayersByName(layerName)
    if len(layer)>1:
        content=['invalid layer name (not unique)']
    else:
        selection = layer[0].selectedFeatures()
        content = [feature[field] for feature in selection]
    return content

Answer 3

One can also apply the concatenate() function:

string_to_array(
    concatenate(
        to_string("id"),
        filter:=is_selected(),
        concatenator:=','
        ),
    delimiter:=',')

Answer 4

Alternatively, one may try using a short PyQGIS code:

layer = iface.activeLayer()
selection = layer.selectedFeatures()
ids = [feature['id'] for feature in selection]

Answer 5

Another possible expression:

array_filter(
    array_foreach(generate_series(0, layer_property(@layer, 'feature_count')-1),
        if(is_selected(get_feature_by_id(@layer, @element)), attribute(get_feature_by_id(@layer, @element), 'id'),'***')
        ),
    @element != '***')

Answer 6

Or you can use Python in the Field Calculator via the Function Editor

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args='auto', group='Custom')
def ids(field, feature, parent):
    layer = iface.activeLayer()
    selection = layer.selectedFeatures()
    ids = [feature[field] for feature in selection]
    return ids

and get the output

Note: That function ids('id') reads field with a single parenthesis i.e. '.