How to use joinFieldNamesSubset from QgsVectorJoinInfo class

Question

I have written a small python plugin to refresh a join on a point layer in QGIS.

point_layer=None
for lyr1 in QgsMapLayerRegistry.instance().mapLayers().values():
  if lyr1.name() == "point_layer":
    point_layer = lyr1

text_table=None
for lyr2 in QgsMapLayerRegistry.instance().mapLayers().values():
if lyr2.name() == "text_table":
  text_table = lyr2

POINT_ID='ID'
TEXT_ID='ID'
joinObject = QgsVectorJoinInfo()
joinObject.joinLayerId = text_table.id()
QgsVectorLayer.removeJoin(point_layer, text_table.id())
joinObject.joinFieldName = TEXT_ID
joinObject.targetFieldName = POINT_ID
joinObject.memoryCache = True
point_layer.addJoin(joinObject)

I tried to get a subset of fields to be used from joined layer text_layer.

joinObject.joinFieldNamesSubset = 'TYPE'

There is no error, but it joins all the fields, not only the field 'TYPE' (attribute of layer text_layer).

How can I set the attributes I want to join? And where do I have to call joinFieldNamesSubset?

Answer 1

Try using the following line instead:

joinObject.setJoinFieldNamesSubset(['TYPE'])

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And as it is a list, you can specify which fields you only want to have joined:

joinObject.setJoinFieldNamesSubset(['Field_1', 'Field_2', 'Field_3'])