Make serializing to different formats trivial

By adding formats to `Serializer::MIME_TYPES` and a `to_#{format}`.

README.md

@@ -127,6 +127,36 @@ appropriate `If-Modified-Since` or `If-None-Match` headers.
 
 [cg]: http://www.rubydoc.info/github/rack/rack/Rack/ConditionalGet
 
+## Different Formats
+
+Although Granola out of the box only ships with JSON serialization support, it's
+easy to extend and add support for different types of serialization in case your
+API needs to provide multiple formats. For example, in order to add MsgPack
+support (via the [msgpack-ruby][] library), you'd do this:
+
+``` ruby
+require "msgpack"
+
+class BaseSerializer < Granola::Serializer
+  MIME_TYPES[:msgpack] = "application/x-msgpack".freeze
+
+  def to_msgpack(*)
+    MsgPack.pack(serialized)
+  end
+end
+```
+
+Now all serializers that inherit from `BaseSerializer` can be serialized into
+MsgPack. In order to use this from our Rack helpers, you'd do:
+
+``` ruby
+granola(object, as: :msgpack)
+```
+
+This will set the correct MIME type.
+
+[msgpack-ruby]: https://github.com/msgpack/msgpack-ruby
+
 ## License
 
 This project is shared under the MIT license. See the attached LICENSE file for

lib/granola.rb

@@ -27,6 +27,10 @@ class << self
   class Serializer
     attr_reader :object
 
+    # Public: Map of the default MIME type for each given type of serialization
+    # for this object.
+    MIME_TYPES = { json: "application/json".freeze }
+
     # Public: Instantiates a list serializer that wraps around an iterable of
     # objects of the type expected by this serializer class.
     #
@@ -70,9 +74,11 @@ def to_json(**options)
     # this will be `application/json`, but you can override in your serializers
     # if your API uses a different MIME type (e.g. `application/my-app+json`).
     #
+    # type - A Symbol describing the expected mime type.
+    #
     # Returns a String.
-    def mime_type
-      "application/json".freeze
+    def mime_type(type = :json)
+      MIME_TYPES.fetch(type)
     end
   end
 

lib/granola/rack.rb

@@ -23,19 +23,22 @@ def self.included(base)
   # object - An object to serialize into JSON.
   #
   # Keywords:
-  #   with:           A specific serializer class to use. If this is `nil`,
-  #                   `Helper.serializer_class_for` will be used to infer the
-  #                   serializer class.
-  #   status:         The HTTP status to return on stale responses. Defaults to
-  #                   `200`.
-  #   headers:        A Hash of default HTTP headers. Defaults to an empty Hash.
-  #   **json_options: Any other keywords passed will be forwarded to the
-  #                   serializer's `#to_json` call.
+  #   with:    A specific serializer class to use. If this is `nil`,
+  #            `Helper.serializer_class_for` will be used to infer the
+  #            serializer class.
+  #   as:      A Symbol with the type of serialization desired. Defaults to
+  #            `:json` (and it's the only one available with Granola by default)
+  #            but could be expanded with plugins to provide serialization to,
+  #            for example, MsgPack.
+  #   status:  The HTTP status to return on stale responses. Defaults to `200`.
+  #   headers: A Hash of default HTTP headers. Defaults to an empty Hash.
+  #   **opts:  Any other keywords passed will be forwarded to the serializer's
+  #            serialization backend call.
   #
   # Raises NameError if no specific serializer is provided and we fail to infer
   #   one for this object.
   # Returns a Rack response tuple.
-  def granola(object, with: nil, status: 200, headers: {}, **json_options)
+  def granola(object, with: nil, status: 200, headers: {}, as: :json, **opts)
     serializer = serializer_for(object, with: with)
 
     if serializer.last_modified
@@ -46,9 +49,9 @@ def granola(object, with: nil, status: 200, headers: {}, **json_options)
       headers["ETag".freeze] = Digest::MD5.hexdigest(serializer.cache_key)
     end
 
-    headers["Content-Type".freeze] = serializer.mime_type
+    headers["Content-Type".freeze] = serializer.mime_type(as)
 
-    body = Enumerator.new { |y| y << serializer.to_json(json_options) }
+    body = Enumerator.new { |y| y << serializer.public_send(:"to_#{as}", opts) }
 
     [status, headers, body]
   end

test/different_format_test.rb

@@ -0,0 +1,40 @@
+require "yaml"
+require "granola/rack"
+
+class BaseSerializer < Granola::Serializer
+  MIME_TYPES[:yaml] = "application/x-yaml".freeze
+
+  def to_yaml(**opts)
+    YAML.dump(serialized)
+  end
+end
+
+User = Struct.new(:name, :age)
+
+class UserSerializer < BaseSerializer
+  def serialized
+    { "name" => object.name, "age" => object.age }
+  end
+end
+
+class Context
+  include Granola::Rack
+end
+
+prepare do
+  @user = User.new("John Doe", 25)
+end
+
+setup { Context.new }
+
+test "allows rendering as a different format" do |context|
+  status, headers, body = context.granola(@user, as: :yaml)
+
+  assert_equal 200, status
+  assert_equal "application/x-yaml", headers["Content-Type"]
+
+  assert_equal(
+    { "name" => "John Doe", "age" => 25 },
+    YAML.load(body.to_a.first)
+  )
+end