Add support for custom mount points in Page::UrlPath (AlchemyCMS#1921)

If an app mounts Alchemy at a non-root path we need to return that as prefix.

Closes AlchemyCMS#1918

app/models/alchemy/page/url_path.rb

@@ -20,8 +20,6 @@ class Page
     #     link_to page.url
     #
     class UrlPath
-      ROOT_PATH = "/"
-
       def initialize(page)
         @page = page
         @language = @page.language
@@ -41,23 +39,27 @@ def call
       private
 
       def language_root_path
-        @language.default? ? ROOT_PATH : language_path
+        @language.default? ? root_path : language_path
       end
 
       def page_path_with_language_prefix
         @language.default? ? page_path : language_path + page_path
       end
 
       def page_path_with_leading_slash
-        @page.language_root? ? ROOT_PATH : page_path
+        @page.language_root? ? root_path : page_path
       end
 
       def language_path
-        "/#{@page.language_code}"
+        "#{root_path}#{@page.language_code}"
       end
 
       def page_path
-        "/#{@page.urlname}"
+        "#{root_path}#{@page.urlname}"
+      end
+
+      def root_path
+        Engine.routes.url_helpers.root_path
       end
     end
   end

spec/models/alchemy/page/url_path_spec.rb

@@ -65,4 +65,20 @@
       end
     end
   end
+
+  context "mounted on a non-root path" do
+    let(:page) do
+      create(:alchemy_page)
+    end
+
+    before do
+      expect(Alchemy::Engine.routes).to receive(:url_helpers) do
+        double(root_path: "/pages/")
+      end
+    end
+
+    it "prefixes the path" do
+      is_expected.to eq("/pages/#{page.urlname}")
+    end
+  end
 end