QGIS - Set layers substring filter with WHERE clause depending on map canvas extent and intersected polygons

set_layer_filter_by_extent.py

class filterLayersFromExtent:
    
    polygon_layer = {
        'name': 'polygons',
        'field': 'code',
        'layer': None
    }
    target_layers = [
        {
            'name': 'observation',
            'field': 'departement',
            'layer': None
        }
    ]
    
    def __init__(self):
        # Set layers
        p = QgsProject.instance()
        
        # QGIS polygon layer
        self.polygon_layer['layer'] = p.mapLayersByName(
            self.polygon_layer['name']
        )[0]
        
        # QGIS target layers
        for layer in self.target_layers:
            layer['layer'] = p.mapLayersByName(
                layer['name']
            )[0]
        
        self.canvas = iface.mapCanvas()
        self.canvas.extentsChanged.connect(self.onCanvasExtentsChanged)

    def getSubsetString(self, extent, layer):
        # Request polygon layer data in the map extent
        request = QgsFeatureRequest(extent)
        request.setFlags(QgsFeatureRequest.ExactIntersect)
        features = self.polygon_layer['layer'].getFeatures(request)
        
        # Get ids of the found features
        ids = []
        field = self.polygon_layer['field']
        for feature in features:
            ids.append(feature[field])
            
        # Compute subset string filter
        target_field = layer['field']
        ids_string = "'" + ("', '").join(ids) + "'"
        
        return f'"{target_field}" IN ( {ids_string} )'

    def onCanvasExtentsChanged(self):
        
        # Get polygons in canvas
        extent = self.canvas.extent()
        
        # For each target layer
        for target_layer in self.target_layers:
            # Get the subset string
            subset_string = self.getSubsetString(extent, target_layer)
            print(subset_string)
            
            # Apply the subset string to the layer
            layer = target_layer['layer']
            if layer and layer.subsetString() != subset_string:
                print(f"{target_layer['name']}: subset string modified !")
                layer.setSubsetString(subset_string)
            
        
test = filterLayersFromExtent()