Add has_one association for root page (AlchemyCMS#1820)

This should allow us to preload better, and I believe Rails will also
cache the association if necessary.

app/models/alchemy/language.rb

@@ -30,6 +30,8 @@ class Language < BaseRecord
 
     before_validation :set_locale, if: -> { locale.blank? }
 
+    has_one :root_page, -> { where(parent: nil) }, class_name: "Alchemy::Page"
+
     validates :name, presence: true
     validates :page_layout, presence: true
     validates :frontpage_name, presence: true
@@ -105,11 +107,6 @@ def label(attrib)
 
     include Alchemy::Language::Code
 
-    # Root page
-    def root_page
-      @root_page ||= pages.language_roots.first
-    end
-
     # All available locales matching this language
     #
     # Matching either the code (+language_code+ + +country_code+) or the +language_code+

spec/models/alchemy/language_spec.rb

@@ -5,6 +5,7 @@
 module Alchemy
   describe Language do
     it { is_expected.to have_many(:nodes) }
+    it { is_expected.to have_one(:root_page) }
 
     let(:default_language) { create(:alchemy_language) }
     let(:language)         { create(:alchemy_language, :klingon) }