QGIS » QGIS Application

# coding: utf-8

from qgis.core import (

    QgsVectorLayerJoinInfo, QgsVectorLayer, QgsField, QgsFeature,

    edit, QgsProject)

from PyQt5.QtCore import QVariant

# Create layer_1

layer_1 = QgsVectorLayer('Point?crs=epsg:21781', 'layer_1', 'memory')

with edit(layer_1):

    layer_1.addAttribute(QgsField('id', QVariant.Int))

    layer_1.addAttribute(QgsField('column_a', QVariant.String))

    layer_1.addAttribute(QgsField('column_b', QVariant.String))

    feat = QgsFeature()

    feat.setAttributes([1, 'A1', 'B1'])

    layer_1.addFeature(feat)

# add layer to the legend

QgsProject.instance().addMapLayer(layer_1)

# Create layer_2

layer_2 = QgsVectorLayer('Point?crs=epsg:21781', 'layer_2', 'memory')

with edit(layer_2):

    layer_2.addAttribute(QgsField('id', QVariant.Int))

    feat = QgsFeature()

    feat.setAttributes([1])

    layer_2.addFeature(feat)

# add layer to the legend

QgsProject.instance().addMapLayer(layer_2)

# Join the layers with the subset filds in the same order than the joined

# layer

join = QgsVectorLayerJoinInfo()

join.setJoinLayer(layer_1)

join.setJoinFieldName('id')

join.setTargetFieldName('id')

join.setJoinFieldNamesSubset(['column_a', 'column_b'])

layer_2.addJoin(join)

# Verify the result

assert next(layer_2.getFeatures()).attribute(

    'layer_1_column_a') == 'A1', 'Join (same order) Failed'

# Remove the join

layer_2.removeJoin(layer_1.id())

# Join the layers with the subset filds in a different order than the joined

# layer

join = QgsVectorLayerJoinInfo()

join.setJoinLayer(layer_1)

join.setJoinFieldName('id')

join.setTargetFieldName('id')

join.setJoinFieldNamesSubset(['column_b', 'column_a'])

layer_2.addJoin(join)

# Verify the result

assert next(layer_2.getFeatures()).attribute(

    'layer_1_column_a') == 'A1', 'Join (different order) Failed'