Filter layer by FID with PyQGIS

Question

I would like to duplicate a layer and apply a filter on the new layer that contains only selected features.

Requirements are:

• using a memory layer is not an option, I want to keep the original datasource
• I want to use a filter, so later changes should not be an issue

How can I use setSubsetString() to execute a query that refers to the internal fid obtained from feature.id()?

Here is my attempt (which doesn't work, because obviously there is no column named "fid"):

lyr = self.cb_layer.currentLayer()
        
# copy layer and apply style
iface.setActiveLayer(lyr)
iface.actionCopyLayerStyle().trigger()
lyr_copy = iface.addVectorLayer(lyr.source(), lyr.name() + '_' + str('temp'), lyr.providerType())
iface.actionPasteLayerStyle().trigger()
        
# create list with ids from selected features 
sel_f = lyr.selectedFeatures()
l = []
for f in sel_f:
   l.append(f.id())
            
query = f"fid in ({tuple(l)})"
         
# Filter
lyr_copy.setSubsetString(query)

Is there an alternative method to setSubsetString() to filter features by the internal id set by QGIS?

Answer 1

Perhaps the strategy is promising to first obtain the unique identifier column for your source layer (on the data provider side) and then filter for the attribute values in this column, in your example:

lyr = self.cb_layer.currentLayer()
# get the key column
key_col = lyr.dataProvider().uri().keyColumn()

if not key_col == '':
    # copy layer and apply style
    iface.setActiveLayer(lyr)
    iface.actionCopyLayerStyle().trigger()
    lyr_copy = iface.addVectorLayer(lyr.source(), lyr.name() + '_' + str('temp'), lyr.providerType())
    iface.actionPasteLayerStyle().trigger()
        
    # create list with ids from selected features 
    sel_f = lyr.selectedFeatures()
    l = []
    for f in sel_f:
        l.append(f[key_col])
    # shorter version to save lines ;-)
    # l = [f[key_col] for f in lyr.selectedFeatures()]

query = f"{key_col} in {tuple(l)}" #leave away the roud brackets around the tuple (cp. below)!
         
# Filter
lyr_copy.setSubsetString(query)

Note(1) that this will only work for layers the have a unique identifier column at all.

Note(2) f"id in ({tuple(...)})" -> "id in ((...))" which is a syntax error.