Add option to select adapter by the request header

README.md

@@ -51,16 +51,36 @@ end
 
 Generally speaking, you as a user of AMS will write (or generate) these
 serializer classes. If you want to use a different adapter, such as a JsonApi, you can
-change this in an initializer:
+change this in your render, or in an initializer:
 
 ```ruby
+#class
 ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::JsonApi
-```
 
+#symbol
+ActiveModel::Serializer.config.adapter = :json_api
+```
 or
 
 ```ruby
-ActiveModel::Serializer.config.adapter = :json_api
+render json: @posts, adapter: :json_api
+```
+
+You can use `Accept` HTTP header-field to select a specific adapter by enabling it.
+
+```ruby
+ActiveModel::Serializer.config.enabled_adapters_by_media_type = true
+```
+By default, AMS responds to media type `application/json` and `application/vnd.api+json` using the `FlattenJson` and `JsonApi` adapters respectively. If you want to add a different media type,  override the defaults or even create your own, you can do it in your initializer.
+
+To override media type `application/json` to respond with `Json` instead of `FlattenJson` adapter:
+```ruby
+ActiveModel::Serializer.config.custom_media_type_adapters = {"application/json" => :json}
+```
+
+If you have a custom adapter (`xml`, as example) you can create your own media type:
+```ruby
+ActiveModel::Serializer.config.custom_media_type_adapters = {"application/xml" => :xml}
 ```
 
 You won't need to implement an adapter unless you wish to use a new format or

lib/action_controller/serialization.rb

@@ -25,6 +25,12 @@ def get_serializer(resource, options = {})
           "Please pass 'adapter: false' or see ActiveSupport::SerializableResource#serialize"
         options[:adapter] = false
       end
+
+      if ActiveModel::Serializer.enabled_adapters_by_media_type && !options.fetch(:adapter, nil)
+        adapter = ActiveModel::Serializer::Adapter.by_request(request)
+        options[:adapter] ||= adapter if adapter
+      end
+
       ActiveModel::SerializableResource.serialize(resource, options) do |serializable_resource|
         if serializable_resource.serializer?
           serializable_resource.serialization_scope ||= serialization_scope

lib/active_model/serializer.rb

@@ -104,6 +104,14 @@ def self.adapter
       adapter_class
     end
 
+    def self.custom_media_type_adapters
+      config.custom_media_type_adapters
+    end
+
+    def self.enabled_adapters_by_media_type
+      config.enabled_adapters_by_media_type
+    end
+
     def self.root_name
       name.demodulize.underscore.sub(/_serializer$/, '') if name
     end

lib/active_model/serializer/adapter.rb

@@ -11,6 +11,11 @@ class Adapter
 
       attr_reader :serializer
 
+      MEDIA_TYPE_ADAPTERS = {
+        "application/vnd.api+json" => "json_api",
+        "application/json" => "flatten_json"
+      }
+
       def initialize(serializer, options = {})
         @serializer = serializer
         @options = options
@@ -37,6 +42,13 @@ def self.adapter_class(adapter)
         "ActiveModel::Serializer::Adapter::#{adapter_name}".safe_constantize
       end
 
+      def self.by_request(request)
+        return unless request && request.accept
+        custom = ActiveModel::Serializer.custom_media_type_adapters
+        media_type_adapters = MEDIA_TYPE_ADAPTERS.merge(custom)
+        media_type_adapters[request.accept]
+      end
+
       def fragment_cache(*args)
         raise NotImplementedError, 'This is an abstract method. Should be implemented at the concrete adapter.'
       end

lib/active_model/serializer/configuration.rb

@@ -7,6 +7,8 @@ module Configuration
       included do |base|
         base.config.array_serializer = ActiveModel::Serializer::ArraySerializer
         base.config.adapter = :flatten_json
+        base.config.enabled_adapters_by_media_type = true
+        base.config.custom_media_type_adapters = {}
       end
     end
   end

test/action_controller/adapter_selector_test.rb

@@ -18,6 +18,11 @@ def render_skipping_adapter
           @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
           render json: @profile, adapter: false
         end
+
+        def render_selecting_adapter_by_accept_header_field
+          @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+          render json: @profile
+        end
       end
 
       tests AdapterSelectorTestController
@@ -48,6 +53,27 @@ def test_render_skipping_adapter
         get :render_skipping_adapter
         assert_equal '{"attributes":{"name":"Name 1","description":"Description 1","comments":"Comments 1"}}', response.body
       end
+
+      def test_render_using_adapter_selected_by_accept_header_field
+        ActiveModel::Serializer.config.enabled_adapters_by_media_type = true
+        request.headers['Accept'] = "application/vnd.api+json"
+
+        get :render_selecting_adapter_by_accept_header_field
+
+        expected = {
+          data: {
+            id: assigns(:profile).id.to_s,
+            type: "profiles",
+            attributes: {
+              name: "Name 1",
+              description: "Description 1",
+            }
+          }
+        }
+
+        assert_equal expected.to_json, response.body
+      end
+
     end
   end
 end