Add ancestors relationship to page serializer

Useful if you want to serialize the page categorizations
in a breadcrumb for instance.

app/models/alchemy/json_api/page.rb

@@ -36,6 +36,10 @@ def fixed_element_ids
         @_fixed_element_ids ||= fixed_elements.map(&:id)
       end
 
+      def ancestor_ids
+        @_ancestor_ids ||= ancestors.map(&:id)
+      end
+
       private
 
       def element_repository

app/serializers/alchemy/json_api/page_serializer.rb

@@ -24,6 +24,12 @@ class PageSerializer
 
       belongs_to :language, record_type: :language, serializer: ::Alchemy::JsonApi::LanguageSerializer
 
+      has_many :ancestors, record_type: :page, serializer: self do |page|
+        page.ancestors.map do |ancestor|
+          Alchemy::JsonApi::Page.new(ancestor, page_version_type: page.page_version_type)
+        end
+      end
+
       # All public elements of this page regardless of if they are fixed or nested.
       # Used for eager loading and should be used as the +include+ parameter of your query
       has_many :all_elements, record_type: :element, serializer: ELEMENT_SERIALIZER

spec/serializers/alchemy/json_api/page_serializer_spec.rb

@@ -66,5 +66,13 @@
         expect(subject[:language]).to eq(data: { id: page.language_id.to_s, type: :language })
       end
     end
+
+    it "has ancestors relationship" do
+      expect(subject[:ancestors]).to eq(
+        data: [
+          { id: page.parent_id.to_s, type: :page },
+        ],
+      )
+    end
   end
 end