Delete nested elements quickly

This builds an array of all the descendent elements from this element,
and then calls the DeleteElements service class on them. Afterwards it
resets the two associations, so that no callbacks run on in-memory
deleted objects.

app/models/alchemy/element.rb

@@ -70,6 +70,8 @@ class Element < BaseRecord
 
     has_many :contents, dependent: :destroy, inverse_of: :element
 
+    before_destroy :destroy_all_nested_elements
+
     has_many :all_nested_elements,
       -> { order(:position) },
       class_name: "Alchemy::Element",
@@ -347,5 +349,16 @@ def touch_touchable_pages
 
       touchable_pages.each(&:touch)
     end
+
+    def destroy_all_nested_elements
+      deeply_nested_elements = descendent_elements(self).flatten
+      DeleteElements.new(deeply_nested_elements).call
+      nested_elements.reset
+      all_nested_elements.reset
+    end
+
+    def descendent_elements(el)
+      el.all_nested_elements + el.all_nested_elements.map { |el| descendent_elements(el) }
+    end
   end
 end

spec/models/alchemy/element_spec.rb

@@ -968,4 +968,18 @@ module Alchemy
       end
     end
   end
+
+  describe "destroy callbacks" do
+    let(:element) { create(:alchemy_element) }
+    let!(:nested_element_1) { create(:alchemy_element, parent_element: element) }
+    let!(:nested_element_2) { create(:alchemy_element, parent_element: nested_element_1) }
+    let!(:nested_element_3) { create(:alchemy_element, parent_element: nested_element_2) }
+
+    it "destroys all the nested elements quickly" do
+      expect(Alchemy::DeleteElements).to receive(:new).with(
+        [nested_element_1, nested_element_2, nested_element_3]
+      ).and_call_original
+      element.reload.destroy!
+    end
+  end
 end