Change Content-Type?

README.md

@@ -219,6 +219,20 @@ end
 If you find that your project is already relying on the old rails to_json
 change `render :json` to `render json: @your_object.to_json`.
 
+# Custom MIME Types
+
+If you're dealing in a custom MIME Type (say, `application/vnd.api+json` or
+similar) and you want `ActiveModel::Serializers` to respond with the correct
+Content-Type header, just pass the `Mime::Type` object to
+`ActiveModel::Serializer.register_mime_type`. If you're registering custom MIME
+Types in a Rails initializer, you'll want to make sure the type is registered
+first, then send it to `ActiveModel::Serializers`.
+
+```ruby
+Mime::Type.register('application/flergle+json', :flergle)
+ActiveModel::Serializer.register_mime_type(Mime::FLERGLE)
+```
+
 # Attributes and Associations
 
 Once you have a serializer, you can specify which attributes and associations

lib/active_model/serializer.rb

@@ -289,6 +289,19 @@ def build_json(controller, resource, options)
 
         serializer.new(resource, options)
       end
+
+      def register_mime_type(mime_type)
+        ActionController.add_renderer(mime_type.symbol) do |resource, options|
+          serialized = ActiveModel::Serializer.build_json(self, resource, options)
+
+          if serialized
+            self.content_type ||= mime_type
+            serialized.to_json
+          else
+            _render_option_json(resource, options)
+          end
+        end
+      end
     end
 
     attr_reader :object, :options

test/serialization_test.rb

@@ -186,6 +186,10 @@ def render_json_empty_array
       render json: []
     end
 
+    def render_custom_mime_type_empty_array
+      render flergle: []
+    end
+
     def render_json_array_with_custom_array_serializer
       render json: [], serializer: CustomArraySerializer
     end
@@ -373,6 +377,15 @@ def test_render_json_empty_array
     assert_equal '{"test":[]}', @response.body
   end
 
+  def test_render_custom_mime_type_empty_array
+    Mime::Type.register('application/flergle+json', :flergle)
+    ActiveModel::Serializer.register_mime_type(Mime::FLERGLE)
+
+    get :render_custom_mime_type_empty_array
+    assert_equal '{"test":[]}', @response.body
+    assert_equal 'application/flergle+json', @response.content_type
+  end
+
   def test_render_json_empty_array_checking_default_root
     get :render_json_empty_array, check_default_root: true
     assert_equal '{"awesome":[]}', @response.body