* Configure adapter using ActiveModel::Serializer.config.adapter

* Get adapter instance using ActiveModel::Serializer::Adapter.adapter_for(serializer)

README.md

@@ -57,7 +57,13 @@ by AMS. If you want to use a different adapter, such as a HalAdapter, you can
 change this in an initializer:
 
 ```ruby
-ActiveModel::Serializer.default_adapter = ActiveModel::Serializer::Adapter::HalAdapter
+ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::HalAdapter
+```
+
+or
+ 
+```ruby
+ActiveModel::Serializer.config.adapter = :hal
 ```
 
 You won't need to implement an adapter unless you wish to use a new format or

lib/action_controller/serialization.rb

@@ -12,7 +12,7 @@ module ActionController
       if serializer
         # omg hax
         object = serializer.new(resource)
-        adapter = ActiveModel::Serializer::Adapter::SimpleAdapter.new(object)
+        adapter = ActiveModel::Serializer::Adapter.adapter_for(object)
 
         super(adapter, options)
       else

lib/active_model/serializer/adapter.rb

@@ -16,6 +16,24 @@ module ActiveModel
       def to_json(options = {})
         raise NotImplementedError, 'This is abstract method. Should be implemented at concrete adapter.'
       end
+
+      def self.adapter_for(serializer)
+        adapter_class = case serializer.config.adapter
+                        when Symbol
+                          class_name = "ActiveModel::Serializer::Adapter::#{serializer.config.adapter.to_s.classify}Adapter"
+                          if Object.const_defined?(class_name)
+                            Object.const_get(class_name)
+                          end
+                        when Class
+                          serializer.config.adapter
+                        end
+        unless adapter_class
+          valid_adapters = self.constants.map { |klass| ":#{klass.to_s.sub('Adapter', '').downcase}" }
+          raise ArgumentError, "Unknown adapter: #{serializer.config.adapter}. Valid adapters are: #{valid_adapters}"
+        end
+
+        adapter_class.new(serializer)
+      end
     end
   end
 end

lib/active_model/serializer/configuration.rb

@@ -6,6 +6,7 @@ module ActiveModel
 
       included do |base|
         base.config.array_serializer = ActiveModel::Serializer::ArraySerializer
+        base.config.adapter = :simple
       end
     end
   end

test/adapter_test.rb

@@ -21,5 +21,54 @@ module ActiveModel
         end
       end
     end
+
+    class AdapterForTest < Minitest::Test
+      def setup
+        profile = Profile.new
+        @serializer = ProfileSerializer.new(profile)
+        @previous_adapter = ActiveModel::Serializer.config.adapter
+      end
+
+      def teardown
+        ActiveModel::Serializer.config.adapter = @previous_adapter
+      end
+
+      def test_returns_default_adapter
+        adapter = Adapter.adapter_for(@serializer)
+        assert_kind_of ActiveModel::Serializer::Adapter::SimpleAdapter, adapter
+      end
+
+      def test_overwrite_adapter_with_symbol
+        ActiveModel::Serializer.config.adapter = :null
+
+        adapter = Adapter.adapter_for(@serializer)
+        assert_kind_of ActiveModel::Serializer::Adapter::NullAdapter, adapter
+      ensure
+
+      end
+
+      def test_overwrite_adapter_with_class
+        ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::NullAdapter
+
+        adapter = Adapter.adapter_for(@serializer)
+        assert_kind_of ActiveModel::Serializer::Adapter::NullAdapter, adapter
+      end
+
+      def test_raises_exception_if_invalid_symbol_given
+        ActiveModel::Serializer.config.adapter = :unknown
+
+        assert_raises ArgumentError do
+          Adapter.adapter_for(@serializer)
+        end
+      end
+
+      def test_raises_exception_if_it_does_not_know_hot_to_infer_adapter
+        ActiveModel::Serializer.config.adapter = 42
+
+        assert_raises ArgumentError do
+          Adapter.adapter_for(@serializer)
+        end
+      end
+    end
   end
 end

test/serializers/configuration_test.rb

@@ -6,6 +6,10 @@ module ActiveModel
       def test_array_serializer
         assert_equal ActiveModel::Serializer::ArraySerializer, ActiveModel::Serializer.config.array_serializer
       end
+
+      def test_adapter
+        assert_equal :simple, ActiveModel::Serializer.config.adapter
+      end
     end
   end
 end