Showing only features related to selected feature in another layer using QGIS

Question

I have: ways (tracks) and points (way signs). The tracks are outgoing from the way signs.

This should be the visible result: The ways with the name 'A' are visible (coloured) because the related way sign 'A' is selected. This is the result I'm looking for:

The question is: How do I show or highlight only the tracks, related to the selected way sign?

What I did: With this rule in styling settings, I can realize the wanted result with one layer (waysigs OR tracks): I select the way sign /tracks and it becomes red because I defined it as a styling rule.

Then I tried this for the tracks (without understanding, what I'm doing) related to the example syntax in the expression builder Help section¹:

    is_selected(get_feature('waysigns', 'fid', "name"), 'waysigns')

but without the expected result...

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¹ s_selected(get_feature('streets', 'name', "street_name"), 'streets') → True if the current building's street is selected.

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Why do I need this: In my project I have thousands of tracks and some way signs. A lot of the tracks are crossing each other and it is impossible, to see, which track comes from which way sign. To have a better overview, of which tracks are related to which way sign, I simply would like to select the way sign and get its related tracks.

Answer 1

Solution 1: Flashing the related features

Run this script in QGIS Python Editor. I couldn't solve how to disable flashing when another point was selected.

w_lyr = QgsProject.instance().mapLayersByName("way_signs")[0]
t_lyr = QgsProject.instance().mapLayersByName("tracts")[0]
f1, f2 = 'fid', 'name'
flashes, duration = 3, 3000 # 3 times, 3 second

def highlight(ids):
    if len(ids)==0:
        return
        
    w = w_lyr.getFeature(ids[0])
    r = f2 + " = '" + str(w[f1]) + "'" #r: request
    
    t_ids = [t.id() for t in t_lyr.getFeatures(r) 
                        if w[f1]==t[f2] and t.geometry().intersects(w.geometry())]
    
    iface.mapCanvas().flashFeatureIds(t_lyr, t_ids,
                                      flashes=flashes,
                                      duration=duration)

w_lyr.selectionChanged.connect(highlight)

Before you run the script again, be sure you disconnected the function using:

w_lyr.selectionChanged.disconnect(highlight)

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Solution 2: Selecting the related features

Using a custom function, you can solve the issue. Add the next script as a new function in the "Function Editor" tab.

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

@qgsfunction(args='auto', group='Custom', usesgeometry=True)
def select_related(point_layer_name, field1, field2, feature, parent):
    point_layer = QgsProject.instance().mapLayersByName(point_layer_name)[0]
    point = point_layer.selectedFeatures()[0]
    if point[field1]== feature[field2] and feature.geometry().intersects(point.geometry()):
        return True

Click "Save and Load Functions". Then, in the "Expression" tab, enter this line:

select_related('way_signs', 'fid', 'name')

There must be a solution which uses only built-in functions, but I didn't figure it out.

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Solution 3: would be creating a temporary layer containing the related features.

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Solution 4: would be somehow styling the related features by means of the "Layer Styling" panel like you try to do.

Answer 2

You can achieve this using a virtual field on the point layer, and a virtual layer.

1. Add a virtual field to the point layer, of type integer size 1 and populate it using the expression is_selected()

(note: don't use a boolean type because of this bug.)

`

So when a point is selected, the value in this field will be set to 1.

2. Create a virtual layer that will join the point layer to the line layer base on their geometry intersection. Then filter out the points to keep only the selected one.

At last, style this virtual layer as you wish.

When you select a point, you will have to pan the map a bit to refresh the virtual layer.

Go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query. Replace the layer name for yours

select l.*
from lines l 
  join points p 
    on st_intersects(p.geometry,l.geometry)
where  p.selected = 1