Performing specific table joins in QGIS via python?

Question

I am trying to update an existing field on a shapefile using data from another shapefile. Basically, each attribute table contains an ID field that I want to use to update a field called "address".

So far, I have tried to use the processing alg "qgis:joinattributestable" and QgsVectorJoinInfo(). Both of these solutions however seem to create temporary join fields that are of no use to me.

Is this something that is possible with QGIS?

Answer 1

You could use something like the following which:

1. Joins your shapefiles using QgsVectorJoinInfo()
2. Updates the address field
3. Removes the join

---

# Change names to match your layer names
layer_1_name = str("layer_1")
layer_2_name = str("layer_2")
for layer in QgsMapLayerRegistry.instance().mapLayers().values():
 if layer.name() == layer_1_name:
 qgis.utils.iface.setActiveLayer(layer)
 layer_1 = qgis.utils.iface.activeLayer() 
 if layer.name() == layer_2_name:
 qgis.utils.iface.setActiveLayer(layer)
 layer_2 = qgis.utils.iface.activeLayer()
# Set up join parameters 
layer_1_Field='ID'
layer_2_Field='ID'
joinObject = QgsVectorJoinInfo()
joinObject.joinLayerId = layer_2.id()
joinObject.joinFieldName = layer_2_Field
joinObject.targetFieldName = layer_1_Field
joinObject.memoryCache = True
layer_1.addJoin(joinObject)
# Define original field and joined field to update
original_field = layer_1.fieldNameIndex('address') 
joined_field = layer_1.fieldNameIndex(layer_2_name + '_address')
layer_1.startEditing()
for feat in layer_1.getFeatures():
 layer_1.changeAttributeValue(feat.id(), original_field, feat.attributes()[joined_field])
# Save the changes
layer_1.commitChanges()
# Remove join 
layer_1.removeJoin(layer_2.id())

---

The code was adapted from an earlier answer .

• JamieTasker: Thanks a bunch for that, worked without a hitch. Is there a way to get it to work using layers opened with 'QgsVectorLayer()'? I've tried to assign my layers as follows but the join does not seem to work: ' Layer 1 = QgsVectorLayer("path", "name". "ogr) Layer 2 = QgsVectorLayer("path","name,"ogr") Thanks
• Joseph: @JamieTasker - Most welcome! You defined the layers but did you load them using QgsMapLayerRegistry.instance().addMapLayers([layer1, layer2])?
• JamieTasker: Excellent, that's exactly what I was after! Thanks ever so much for your help!
• Joseph: @JamieTasker - Glad it was helpful ;)
• grego: For anyone reading this using QGIS 3.x : QgsMapLayerRegistry is gone from QGIS 3.x; it's functionality has moved to QgsProject. So you would use: for layer in QgsProject.instance().mapLayers().values(): instead of for layer in QgsMapLayerRegistry.instance().mapLayers().values():