Set expression in QGIS edit widget relation reference with PyQGIS

Question

I've setup an edit widget for a field as a relation reference like shown below and save the configuration as a QGIS layer style file (*.qml):

With PyQGIS I do the following steps

• load the vector layer necessary for the relations
• create the relations
• load the style files

But after this, the expression dropdown is empty because it is not stored in the qml file:

<edittype widgetv2type="RelationReference" name="nv_bezeichnung">
   <widgetv2config OrderByValue="0" fieldEditable="1" ShowForm="0" Relation="rv-nv" ReadOnly="0" MapIdentification="1" labelOnTop="1" AllowNULL="0"/>
</edittype>

The edit widgets can be configured with PyQGIS like

layer.setEditorWidgetV2Config(fieldId, {'key':'value'})

with keys according to the xml snippet above, but there seems to be no key to store the expression in, therefore my question:

How to store or save the 'Display Expression' in a relation reference or set it up using PyQGIS?

Answer 1

This problem has quite a simple solution, which is not so obvious at first sight. I found it by studying the documentation to QgsVectorLayer in detail. This has a method setDisplayExpression(). The solution is that the display expression is not a property of the child layer for which the relation reference is set up, but for it's parent layer (the 1 in the 1:n relation).

So in PyQGIS the Display expression for the child layer Rohrverbund is set up far its parent layer Netzverteiler via

QgsMapLayerRegistry.instance().mapLayersByName('Netzverteiler').setDisplayExpression('bezeichnung')

where attribute bezeichnung appears in the combo box where parent layer is selected.