Add subscription previewing

lib/recurly/subscription.rb

@@ -2,6 +2,8 @@ module Recurly
   class Subscription < Resource
     autoload :AddOns, 'recurly/subscription/add_ons'
 
+    class NotPreviewableError < StandardError; end
+
     # @macro [attach] scope
     #   @scope class
     #   @return [Pager<Subscription>] A pager that yields +$1+ subscriptions.
@@ -51,6 +53,20 @@ class Subscription < Resource
     )
     alias to_param uuid
 
+    def self.preview(attributes = {})
+      new(attributes) { |record| record.preview }
+    end
+
+    def preview
+      raise NotPreviewableError.new('Cannot preview an existing subscription') unless new_record?
+
+      clear_errors
+      @response = API.send(:post, "#{path}/preview", to_xml)
+      reload response
+    rescue API::UnprocessableEntity => e
+      apply_errors e
+    end
+
     # @return [Subscription] A new subscription.
     def initialize attributes = {}
       super({ :currency => Recurly.default_currency }.merge attributes)

spec/recurly/subscription_spec.rb

@@ -251,6 +251,16 @@
         subscription.invoice.must_equal nil
       end
     end
+  end
+
+  describe 'previewing' do
+    it 'cannot preview an existing subscription' do
+      stub_api_request :get, 'subscriptions/abcdef1234567890', 'subscriptions/show-200-noinvoice'
 
+      subscription = Subscription.find 'abcdef1234567890'
+      assert_raises Subscription::NotPreviewableError do
+        subscription.preview
+      end
+    end
   end
 end