Nullify referencing EssencePages on page deletion

It's impossible to delete a page if it is referenced from an
EssencePage. Currently, we get a ForeignKeyConstraint error if we intend
to do this; but it would be much nicer if the essence would stay where
it is and stop referencing the page.

app/models/alchemy/page.rb

@@ -121,6 +121,8 @@ class Page < BaseRecord
     has_one :draft_version, -> { drafts }, class_name: "Alchemy::PageVersion"
     has_one :public_version, -> { published }, class_name: "Alchemy::PageVersion", autosave: -> { persisted? }
 
+    has_many :page_essences, class_name: "Alchemy::EssencePage", foreign_key: :page_id, inverse_of: :ingredient_association, dependent: :nullify
+
     before_validation :set_language,
       if: -> { language.nil? }
 

spec/models/alchemy/page_spec.rb

@@ -220,6 +220,14 @@ module Alchemy
         it "destroys elements along with itself" do
           expect { page.destroy! }.to change(Alchemy::Element, :count).from(3).to(0)
         end
+
+        context "with an essence page pointing to the page" do
+          let!(:essence_page) { create(:alchemy_essence_page, page: page) }
+
+          it "nullifies the essence on destruction" do
+            expect { page.destroy! }.to change { essence_page.reload.page_id }.from(page.id).to(nil)
+          end
+        end
       end
     end