PyQGIS Set relation strength to "composition"

Question

Working with QGIS 3.34 (ltr) and PyQGIS, I'm trying to set up a relation between point layer and a No Geometry table.

I found out that the correct class to use is: relation.setStrength(QgsRelation.Composition).

The code seems to work, but, when I check on QGIS > Project > Property > Relation, I still get an 'Association' relationship. Do I need to add more relation set up?

Here is my full function:

def create_relation(parent_layer_name, child_layer_name, parent_field, child_field, 
relation_id, relation_name):

# Retrieve layers by name
   parent_layers = QgsProject.instance().mapLayersByName(parent_layer_name)
   child_layers = QgsProject.instance().mapLayersByName(child_layer_name)

   if not parent_layers or not child_layers:
      print(f"Layer '{parent_layer_name}' or '{child_layer_name}' not found.")
      return

    parent_layer = parent_layers[0]
    child_layer = child_layers[0]

   # Debug: Print layer IDs
   print(f"Parent Layer ID: {parent_layer.id()}, Child Layer ID: {child_layer.id()}")

   # Create a new QgsRelation object
   relation = QgsRelation()
   relation.setId(relation_id)
   relation.setName(relation_name)
   relation.setReferencedLayer(parent_layer.id())     # Parent layer
   relation.setReferencingLayer(child_layer.id())     # Child layer

   # Define the field pair (foreign key and primary key)
   relation.addFieldPair(child_field, parent_field)

   # Set relation strength to composition
   relation.setStrength(QgsRelation.Composition)

   # Validate and add the relation
   if relation.isValid():
      QgsProject.instance().relationManager().addRelation(relation)
      print(f"Relation '{relation_name}' created successfully.")
   else:
      print(f"Relation '{relation_name}' is not valid. Validation error: {relation.validationError()}")

Answer 1

The code I posted in my question is working correctly. I was using a custom .qml file that saves the style for my layer, including the relation as an association. However, I applied this custom style after creating the relation, which removed the "composition." The solution is to create the relation after applying the style.