Do not destroy nodes with dependent nodes that are in use on a page

This moves the validation up in the file so that awesome_nested_set does
not run its `delete_all` callback before we validate that it can do
that.

It also extends the validation such that we collect all the pages that
are referenced in the subtree below this node.

app/models/alchemy/node.rb

@@ -4,6 +4,8 @@ module Alchemy
   class Node < BaseRecord
     VALID_URL_REGEX = /\A(\/|\D[a-z\+\d\.\-]+:)/
 
+    before_destroy :check_if_related_essence_nodes_present
+
     acts_as_nested_set scope: "language_id", touch: true
     stampable stamper_class_name: Alchemy.user_class_name
 
@@ -14,8 +16,6 @@ class Node < BaseRecord
 
     has_many :essence_nodes, class_name: "Alchemy::EssenceNode", foreign_key: :node_id, inverse_of: :ingredient_association
 
-    before_destroy :check_if_related_essence_nodes_present
-
     validates :name, presence: true, if: -> { page.nil? }
     validates :url, format: { with: VALID_URL_REGEX }, unless: -> { url.nil? }
 
@@ -79,8 +79,9 @@ def view_folder_name
     end
 
     def check_if_related_essence_nodes_present
-      if essence_nodes.any?
-        errors.add(:base, :essence_nodes_present, page_names: essence_nodes.map(&:page).map(&:name).to_sentence)
+      dependent_essence_nodes = self_and_descendants.flat_map(&:essence_nodes)
+      if dependent_essence_nodes.any?
+        errors.add(:base, :essence_nodes_present, page_names: dependent_essence_nodes.map(&:page).map(&:name).to_sentence)
         throw(:abort)
       end
     end

spec/models/alchemy/node_spec.rb

@@ -152,6 +152,16 @@ module Alchemy
         expect(node).not_to be_destroyed
         expect(node.errors.full_messages).to eq(["This menu item is in use inside an Alchemy element on the following pages: #{page.name}."])
       end
+
+      context "if there are essence nodes present on a child node" do
+        let!(:parent_node) { create(:alchemy_node, children: [node]) }
+
+        it "does not destroy the node and children either but adds an error" do
+          parent_node.destroy
+          expect(parent_node).not_to be_destroyed
+          expect(parent_node.errors.full_messages).to eq(["This menu item is in use inside an Alchemy element on the following pages: #{page.name}."])
+        end
+      end
     end
   end
 end