From https://once.com/writebook

app/controllers/concerns/page_leaf_scoped.rb

@@ -0,0 +1,10 @@
+module PageLeafScoped extend ActiveSupport::Concern
+  included do
+    before_action :set_leaf
+  end
+
+  private
+    def set_leaf
+      @leaf = Current.user.leaves.find(params[:page_id])
+    end
+end