Support s3 host alias with prefixes

Allows `s3_host_alias` to be set to CDNs which support S3 Bucket prefix.

[fixes #2287]

lib/paperclip/storage/s3.rb

@@ -65,6 +65,9 @@ module Storage
     # * +s3_host_alias+: The fully-qualified domain name (FQDN) that is the alias to the
     #   S3 domain of your bucket. Used with the :s3_alias_url url interpolation. See the
     #   link in the +url+ entry for more information about S3 domains and buckets.
+    # * +s3_prefixes_in_alias+: The number of prefixes that is prepended by
+    #   s3_host_alias. This will remove the prefixes from the path in
+    #   :s3_alias_url url interpolation
     # * +url+: There are four options for the S3 url. You can choose to have the bucket's name
     #   placed domain-style (bucket.s3.amazonaws.com) or path-style (s3.amazonaws.com/bucket).
     #   You can also specify a CNAME (which requires the CNAME to be specified as
@@ -164,7 +167,13 @@ def self.extended base
         end
 
         Paperclip.interpolates(:s3_alias_url) do |attachment, style|
-          "#{attachment.s3_protocol(style, true)}//#{attachment.s3_host_alias}/#{attachment.path(style).sub(%r{\A/}, "".freeze)}"
+          protocol = attachment.s3_protocol(style, true)
+          host = attachment.s3_host_alias
+          path = attachment.path(style).
+            split("/")[attachment.s3_prefixes_in_alias..-1].
+            join("/").
+            sub(%r{\A/}, "".freeze)
+          "#{protocol}//#{host}/#{path}"
         end unless Paperclip::Interpolations.respond_to? :s3_alias_url
         Paperclip.interpolates(:s3_path_url) do |attachment, style|
           "#{attachment.s3_protocol(style, true)}//#{attachment.s3_host_name}/#{attachment.bucket_name}/#{attachment.path(style).sub(%r{\A/}, "".freeze)}"
@@ -213,6 +222,10 @@ def s3_host_alias
         @s3_host_alias
       end
 
+      def s3_prefixes_in_alias
+        @s3_prefixes_in_alias ||= @options[:s3_prefixes_in_alias].to_i
+      end
+
       def s3_url_options
         s3_url_options = @options[:s3_url_options] || {}
         s3_url_options = s3_url_options.call(instance) if s3_url_options.respond_to?(:call)

spec/paperclip/storage/s3_spec.rb

@@ -567,6 +567,33 @@ def original_filename
     end
   end
 
+  context "generating a url with a prefixed host alias" do
+    before do
+      rebuild_model(
+        aws2_add_region.merge(
+          storage: :s3,
+          s3_credentials: {
+            production: { bucket: "prod_bucket" },
+            development: { bucket: "dev_bucket" },
+          },
+          bucket: "bucket",
+          s3_host_alias: "something.something.com",
+          s3_prefixes_in_alias: 2,
+          path: "prefix1/prefix2/:attachment/:basename:dotextension",
+          url: ":s3_alias_url",
+        )
+      )
+      @dummy = Dummy.new
+      @dummy.avatar = stringy_file
+      @dummy.stubs(:new_record?).returns(false)
+    end
+
+    it "returns a url with the prefixes removed" do
+      assert_match %r{^//something.something.com/avatars/data[^\.]},
+                   @dummy.avatar.url
+    end
+  end
+
   context "generating a url with a proc as the host alias" do
     before do
       rebuild_model (aws2_add_region).merge storage: :s3,