From https://once.com/writebook

app/controllers/concerns/set_book_leaf.rb

@@ -0,0 +1,37 @@
+module SetBookLeaf
+  extend ActiveSupport::Concern
+
+  included do
+    before_action :set_book
+    before_action :set_leaf, :set_leafable, only: %i[ show edit update destroy ]
+  end
+
+  private
+    def set_book
+      @book = Book.accessable_or_published.find(params[:book_id])
+    end
+
+    def set_leaf
+      @leaf = @book.leaves.active.find(params[:id])
+    end
+
+    def set_leafable
+      instance_variable_set "@#{instance_name}", @leaf.leafable
+    end
+
+    def ensure_editable
+      head :forbidden unless @book.editable?
+    end
+
+    def model_class
+      controller_leafable_name.constantize
+    end
+
+    def instance_name
+      controller_leafable_name.underscore
+    end
+
+    def controller_leafable_name
+      self.class.to_s.remove("Controller").demodulize.singularize
+    end
+end