How to join a layer once when toggling visibility?

Question

I have a set of layers contained in several groups ("First group", "Second group" and "Third group"). I created a function where when a layer from any of these groups has its visibility toggled, it is joined by a 'parent' layer which shares the same name as the group the layer is in:

So in the example shown, if I click on Layer_1, it will be joined to the First Group layer. If I click on Layer_8, it will be joined to the Third group layer etc.

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The problem is that when I click on a layer from the "First, Second or Third group", it is joined several times instead of just the once.

For example, if I click Layer_3 once and then check the Joins properties:

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Here is the code:

from PyQt4.QtCore import *
root = QgsProject.instance().layerTreeRoot()
main_group = root.findGroup('Main group')
layerField = 'ID'

def update_joins():
    for group in main_group.children():
        layer = QgsMapLayerRegistry.instance().mapLayersByName( group.name() )[-1]
        for child in group.children():
            node = root.findLayer(child.layer().id()) 
            if node.isVisible() == Qt.Checked:  
                childField = 'ID'
                joinObject = QgsVectorJoinInfo()
                joinObject.joinLayerId = child.layer().id()
                joinObject.joinFieldName = childField
                joinObject.targetFieldName = layerField
                joinObject.memoryCache = True
                layer.addJoin(joinObject) 
            else:
                layer.removeJoin(child.layer().id())

root.visibilityChanged.connect(update_joins)

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I'm not exactly sure why it is being joined several times, is there a way to make sure it is only joined once?

I checked QgsVectorJoinInfo and the QgsVectorLayer::vectorJoins to see if it is possible to do a check (i.e. if this layer has been joined then pass) but couldn't find much information.

Answer 1

Update

Thinking more about this using a dictionary to hold the layer names and the related layer join field name and evaluating each checked layer to see if it has the reference joined field should be a better method. If join field is present, than skip, otherwise perform the join.

from PyQt4.QtCore import *
root = QgsProject.instance().layerTreeRoot()
main_group = root.findGroup('Main group')
layerField = 'ID'
layerFieldRef = {'First group':'FirstGroupLayerFieldName', 
               'Second group':'SecondGroupLayerFieldName', 
               'Second group':'SecondGroupLayerFieldName',                    
               'Second group':'SecondGroupLayerFieldName',   
               'Second group':'SecondGroupLayerFieldName', 
               'Third group':'ThirdGroupLayerFieldName', 
               'Third group':'ThirdGroupLayerFieldName',           
               'Third group':'ThirdGroupLayerFieldName'
                }

def update_joins():
    for group in main_group.children():
        layer = QgsMapLayerRegistry.instance().mapLayersByName( group.name() )[-1]
        for child in group.children():
            node = root.findLayer(child.layer().id()) 
            if node.isVisible() == Qt.Checked:
                # check if related layer join field is present, if not present than join
                fields = layer.pendingFields()
                for key in layerFieldRef:
                    if fields.indexFromName(key + '_' + layerFieldRef[key]) == -1:  
                        childField = 'ID'
                        joinObject = QgsVectorJoinInfo()
                        joinObject.joinLayerId = child.layer().id()
                        joinObject.joinFieldName = childField
                        joinObject.targetFieldName = layerField
                        joinObject.memoryCache = True
                        layer.addJoin(joinObject) 
            else:
                layer.removeJoin(child.layer().id())

root.visibilityChanged.connect(update_joins)

This should eliminate extra joins on a layer that already has a join.