Make sure a changed manager group gets the reviewer role

The manager group of a staging workflow could have lost its review role
of the project attached to a staging workflow. That made updating the
manager group to throw an error.

Remove reviewer role for old managers role and add reviewer role for new
managers role.

src/api/app/models/staging/workflow.rb

@@ -125,18 +125,13 @@ def update_managers_group
     # the object is reloaded and we lost the changes.
     self.managers_group = new_managers_group
 
-    # update reviewer group in staging workflow project
     reviewer_role = Role.find_by_title('reviewer')
-    relationship = project.relationships.find_by(group: old_managers_group, role: reviewer_role)
-    if project.relationships.exists?(group: new_managers_group, role: reviewer_role)
-      relationship.destroy
-    else
-      relationship.update(group: new_managers_group)
-    end
+    project.relationships.find_by(group: old_managers_group, role: reviewer_role)&.destroy   # Remove reviewer role for old managers group
+    project.relationships.find_or_create_by!(group: new_managers_group, role: reviewer_role) # Add reviewer role for new managers group
+
     project.store
   end
 end
-
 # == Schema Information
 #
 # Table name: staging_workflows