Click features & aggregate unique attributes only using QGIS

Question

In QGIS 3.16 the "merge feature attributes" function (on the advanced digitizing toolbar) allows merging the attributes of selected features by either 1) selecting which feature to keep the attributes from, or 2) concatenating the values from all selected features. However, there is no 'concatenate unique' option here; equal values will be duplicated.

Is there a way to concatenate the attribute values without duplication?

I am not looking to do this across the board for all features in a layer; just for features that I click and select.

Answer 1

Nice use case for Layer Actions.

Just configure a new layer action like this:

Code for the layer action:

from qgis.core import QgsProject
from qgis.utils import iface

field_name = "etiqueta"  # CHANGE THIS TO YOUR OWN FIELD NAME

layer = QgsProject.instance().mapLayer("[% @layer_id %]")
idx = layer.fields().indexOf(field_name)

if layer.selectedFeatureCount():
    params = QgsAggregateCalculator.AggregateParameters()
    params.delimiter = ','

    value = layer.aggregate(
        QgsAggregateCalculator.StringConcatenateUnique,
        field_name, params, fids=layer.selectedFeatureIds())[0]
        
    for fid in layer.selectedFeatureIds():
        layer.changeAttributeValue(fid, idx, value)
            
    iface.messageBar().pushSuccess("Concatenate unique", "Done!")
else:
    iface.messageBar().pushInfo("Concatenate unique", "First select some features...")

---

Notes:

• You just need to change the field_name variable to match your field name and you're done.
• The layer action will be available in editing mode. It won't be saved until you click on Save Layer Edits.

---

Below the code for concatenating several fields at once:

from qgis.core import QgsProject
from qgis.utils import iface

field_names = ["my_field_1", "my_field_2"]  # CHANGE THIS TO YOUR OWN FIELD NAMES

layer = QgsProject.instance().mapLayer("[% @layer_id %]")
field_dict = {layer.fields().indexOf(fn): fn for fn in field_names}

if layer.selectedFeatureCount():
    params = QgsAggregateCalculator.AggregateParameters()
    params.delimiter = ','

    attr_map = dict()
    for idx, field_name in field_dict.items():
        value = layer.aggregate(
            QgsAggregateCalculator.StringConcatenateUnique,
            field_name, params, fids=layer.selectedFeatureIds())[0]
        attr_map[idx] = value
        
    for fid in layer.selectedFeatureIds():
        layer.changeAttributeValues(fid, attr_map)
            
    iface.messageBar().pushSuccess("Concatenate unique", "Done!")
else:
    iface.messageBar().pushInfo("Concatenate unique", "First select some features...")

Answer 2

You can run the Aggregate tool and check the box next to Selected features only. Set the Aggregate function to concatenate_unique.