I'm using QGIS 3.10.1. I've created virtual layers from an ODBC data source and using the (relatively) new feature "label editing". I was able to show/hide/move labels of a layer. But just after saving and refreshing/reloading the layer all 'auxiliary_storage_' labeling attribute values are gone. The same happens if I restart the QGIS project. Saving the project as qgz (compressed, including QGD) doesn't provide any remedy.

The workaround would be to add a custom Python action for/at the virtual layer scope, pointing to an expression like this (it will temporarily re-populate all stored feature attributes from the joined, in-memory auxiliary layer to the target, non-editable vector layer), credits go to @Detlev:

# -*- coding: utf-8 -*-

from qgis.core import *
from qgis.gui import *
from qgis.utils import *

@qgsfunction(args='auto', group='My')
def update_aux(feature, parent, context):
    Updates the auxiliary layer's attributes (if any) of the selected layer
    <h2>Example usage:</h2>
    layer = QgsProject.instance().mapLayer(context.variable("layer_id"))
    auxiliaryLayer = layer.auxiliaryLayer()
    if auxiliaryLayer is not None:
        joinField = auxiliaryLayer.joinInfo().joinFieldName()
        joinKey = auxiliaryLayer.joinInfo().targetFieldName()
        QgsMessageLog.logMessage("Updating auxiliary layer({}) values, join:  ({}->{})".format(auxiliaryLayer.id(), joinField, joinKey), "My", level=Qgis.Info)
        fnm = auxiliaryLayer.dataProvider().fieldNameMap()
        fim = {index: name for name, index in fnm.items()}
        for auxiliaryFeature in auxiliaryLayer.getFeatures():
            filterExpression = "{} = {}".format(joinKey, auxiliaryFeature[joinField])
            featureRequest = QgsFeatureRequest().setFilterExpression(filterExpression)
            layerFeature = QgsFeature()
            if layerFeature is not None:
                for i in fim:
                    propertyDefinition = auxiliaryLayer.propertyDefinitionFromIndex(i)
                    auxiliaryFieldName = QgsAuxiliaryLayer.nameFromProperty(propertyDefinition, True)
                    auxiliaryFieldIndex = layer.fields().indexFromName(auxiliaryFieldName)
                    if auxiliaryFieldIndex >= 0:
                        QgsMessageLog.logMessage("changeAttributeValue({}, {}, {})".format(layerFeature.id(), auxiliaryFieldIndex, auxiliaryFeature[i]), "My", level=Qgis.Info)
                        layer.changeAttributeValue(layerFeature.id(), auxiliaryFieldIndex, auxiliaryFeature[i])
                QgsMessageLog.logMessage("Couldn't find a feature with {} in the current layer!".format(featureRequest.filterExpression()), "My", level=Qgis.Info)
        if iface.mapCanvas().isCachingEnabled():
        QgsMessageLog.logMessage("There is no auxiliary layer!", "My", level=Qgis.Warning)

Does somebody have an idea, what's going wrong here? I've spent countless hours formatting the labels using the rule-based engine for printing, it doesn't seem to be a proper solution for map production.

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