Make Adapters registerable so they are not namespace-constrained

Changes:

- Introduce Adapter::get for use by Serializer.adapter
- Move Adapter-finding logic from Adapter::adapter_class into Adapter::get

Introduced interfaces:

- non-inherited methods
```ruby
ActiveModel::Serializer::Adapter.adapter_map     # a Hash<adapter_name, adapter_class>
ActiveModel::Serializer::Adapter.adapters        # an Array<adapter_name>
ActiveModel::Serializer::Adapter.register(name, klass) # adds an adapter to the adapter_map
ActiveModel::Serializer::Adapter.get(name_or_klass)    # raises Argument error when adapter not found
```

- Automatically register adapters when subclassing

```ruby
      def self.inherited(subclass)
        ActiveModel::Serializer::Adapter.register(subclass.to_s.demodulize, subclass)
      end
```

- Preserves subclass method `::adapter_class(adapter)`

```ruby
      def self.adapter_class(adapter)
        ActiveModel::Serializer::Adapter.get(adapter)
      end
```

- Serializer.adapter now uses `Adapter.get(config.adapter)` rather than have duplicate logic

.gitignore

@@ -4,6 +4,7 @@
 .config
 .yardoc
 Gemfile.lock
+Gemfile.local
 InstalledFiles
 _yardoc
 coverage

Gemfile

@@ -1,4 +1,10 @@
 source 'https://rubygems.org'
+#
+# Add a Gemfile.local to locally bundle gems outside of version control
+local_gemfile = File.join(File.expand_path("..", __FILE__), "Gemfile.local")
+if File.readable?(local_gemfile)
+  eval_gemfile local_gemfile
+end
 
 # Specify your gem's dependencies in active_model_serializers.gemspec
 gemspec

docs/general/adapters.md

@@ -49,3 +49,7 @@ If you want to have a root key in your responses you should use the Json adapter
 ```ruby
 ActiveModel::Serializer.config.adapter = :json
 ```
+
+## Registering an adapter
+
+ActiveModel::Serializer::Adapter.register(:my_adapter, MyAdapter)

lib/active_model/serializer.rb

@@ -89,19 +89,9 @@ def self.serializer_for(resource, options = {})
       end
     end
 
+    # @see ActiveModel::Serializer::Adapter.get
     def self.adapter
-      adapter_class = case config.adapter
-                      when Symbol
-                        ActiveModel::Serializer::Adapter.adapter_class(config.adapter)
-                      when Class
-                        config.adapter
-                      end
-      unless adapter_class
-        valid_adapters = Adapter.constants.map { |klass| ":#{klass.to_s.downcase}" }
-        raise ArgumentError, "Unknown adapter: #{config.adapter}. Valid adapters are: #{valid_adapters}"
-      end
-
-      adapter_class
+      ActiveModel::Serializer::Adapter.get(config.adapter)
     end
 
     def self.root_name

lib/active_model/serializer/adapter.rb

@@ -3,6 +3,8 @@
 module ActiveModel
   class Serializer
     class Adapter
+      UnknownAdapterError = Class.new(ArgumentError)
+      ADAPTER_MAP = {}
       extend ActiveSupport::Autoload
       autoload :Json
       autoload :FlattenJson
@@ -11,6 +13,64 @@ class Adapter
 
       attr_reader :serializer
 
+      # Only the Adapter class has these methods.
+      # None of the sublasses have them.
+      class << ActiveModel::Serializer::Adapter
+        # @return Hash<adapter_name, adapter_class>
+        def adapter_map
+          ADAPTER_MAP
+        end
+
+        # @return Array<adapter_name>
+        def adapters
+          adapter_map.keys.sort
+        end
+
+        # Adds an adapter 'klass' with 'name' to the 'adapter_map'
+        # Names are stringified and underscored
+        def register(name, klass)
+          adapter_map.update(name.to_s.underscore => klass)
+          self
+        end
+
+        # @param  adapter [String, Symbol, Class] name to fetch adapter by
+        # @return [ActiveModel::Serializer::Adapter] subclass of Adapter
+        # @raise  [UnknownAdapterError]
+        def get(adapter)
+          # 1. return if is a class
+          return adapter if adapter.is_a?(Class)
+          adapter_name = adapter.to_s.underscore
+          # 2. return if registered
+          adapter_map.fetch(adapter_name) {
+            # 3. try to find adapter class from environment
+            adapter_class = find_by_name(adapter_name)
+            register(adapter_name, adapter_class)
+            adapter_class
+          }
+        rescue ArgumentError
+          failure_message =
+            "Unknown adapter: #{adapter.inspect}. Valid adapters are: #{adapters}"
+          raise UnknownAdapterError, failure_message, $!.backtrace
+        rescue NameError
+          failure_message =
+            "NameError: #{$!.message}. Unknown adapter: #{adapter.inspect}. Valid adapters are: #{adapters}"
+          raise UnknownAdapterError, failure_message, $!.backtrace
+        end
+
+        # @api private
+        def find_by_name(adapter_name)
+          adapter_name = adapter_name.to_s.classify.tr("API", "Api")
+          "ActiveModel::Serializer::Adapter::#{adapter_name}".safe_constantize or
+            fail UnknownAdapterError
+        end
+        private :find_by_name
+      end
+
+      # Automatically register adapters when subclassing
+      def self.inherited(subclass)
+        ActiveModel::Serializer::Adapter.register(subclass.to_s.demodulize, subclass)
+      end
+
       def initialize(serializer, options = {})
         @serializer = serializer
         @options = options
@@ -32,9 +92,9 @@ def self.create(resource, options = {})
         klass.new(resource, options)
       end
 
+      # @see ActiveModel::Serializer::Adapter.get
       def self.adapter_class(adapter)
-        adapter_name = adapter.to_s.classify.sub("API", "Api")
-        "ActiveModel::Serializer::Adapter::#{adapter_name}".safe_constantize
+        ActiveModel::Serializer::Adapter.get(adapter)
       end
 
       def fragment_cache(*args)

test/adapter_test.rb

@@ -19,16 +19,6 @@ def test_serializer
         assert_equal @serializer, @adapter.serializer
       end
 
-      def test_adapter_class_for_known_adapter
-        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_api)
-        assert_equal ActiveModel::Serializer::Adapter::JsonApi, klass
-      end
-
-      def test_adapter_class_for_unknown_adapter
-        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_simple)
-        assert_nil klass
-      end
-
       def test_create_adapter
         adapter = ActiveModel::Serializer::Adapter.create(@serializer)
         assert_equal ActiveModel::Serializer::Adapter::FlattenJson, adapter.class

test/serializers/adapter_for_test.rb

@@ -1,6 +1,8 @@
 module ActiveModel
   class Serializer
     class AdapterForTest < Minitest::Test
+      UnknownAdapterError = ::ActiveModel::Serializer::Adapter::UnknownAdapterError
+
       def setup
         @previous_adapter = ActiveModel::Serializer.config.adapter
       end
@@ -20,7 +22,7 @@ def test_overwrite_adapter_with_symbol
         adapter = ActiveModel::Serializer.adapter
         assert_equal ActiveModel::Serializer::Adapter::Null, adapter
       ensure
-
+        ActiveModel::Serializer.config.adapter = @previous_adapter
       end
 
       def test_overwrite_adapter_with_class
@@ -33,18 +35,114 @@ def test_overwrite_adapter_with_class
       def test_raises_exception_if_invalid_symbol_given
         ActiveModel::Serializer.config.adapter = :unknown
 
-        assert_raises ArgumentError do
+        assert_raises UnknownAdapterError do
           ActiveModel::Serializer.adapter
         end
       end
 
       def test_raises_exception_if_it_does_not_know_hot_to_infer_adapter
         ActiveModel::Serializer.config.adapter = 42
 
-        assert_raises ArgumentError do
+        assert_raises UnknownAdapterError do
           ActiveModel::Serializer.adapter
         end
       end
+
+      def test_adapter_class_for_known_adapter
+        klass = ActiveModel::Serializer::Adapter.adapter_class(:json_api)
+        assert_equal ActiveModel::Serializer::Adapter::JsonApi, klass
+      end
+
+      def test_adapter_class_for_unknown_adapter
+        assert_raises UnknownAdapterError  do
+          ActiveModel::Serializer::Adapter.adapter_class(:json_simple)
+        end
+      end
+
+      def test_adapter_map
+        assert_equal ActiveModel::Serializer::Adapter.adapter_map, {
+          "json".freeze              => ActiveModel::Serializer::Adapter::Json,
+          "json_api".freeze          => ActiveModel::Serializer::Adapter::JsonApi,
+          "flatten_json".freeze      => ActiveModel::Serializer::Adapter::FlattenJson,
+          "null".freeze              => ActiveModel::Serializer::Adapter::Null
+        }
+      end
+
+      def test_adapters
+        assert_equal ActiveModel::Serializer::Adapter.adapters.sort, [
+          "flatten_json".freeze,
+          "json".freeze,
+          "json_api".freeze,
+          "null".freeze,
+        ]
+      end
+
+      def test_get_adapter_by_string_name
+        assert_equal ActiveModel::Serializer::Adapter.get("json".freeze), ActiveModel::Serializer::Adapter::Json
+      end
+
+      def test_get_adapter_by_symbol_name
+        assert_equal ActiveModel::Serializer::Adapter.get(:json), ActiveModel::Serializer::Adapter::Json
+      end
+
+      def test_get_adapter_by_class
+        klass = ActiveModel::Serializer::Adapter::Json
+        assert_equal ActiveModel::Serializer::Adapter.get(klass), klass
+      end
+
+      def test_get_adapter_from_environment_registers_adapter
+        ActiveModel::Serializer::Adapter.const_set(:AdapterFromEnvironment, Class.new)
+        klass = ::ActiveModel::Serializer::Adapter::AdapterFromEnvironment
+        name = "adapter_from_environment".freeze
+        assert_equal ActiveModel::Serializer::Adapter.get(name), klass
+        assert ActiveModel::Serializer::Adapter.adapters.include?(name)
+      ensure
+        ActiveModel::Serializer::Adapter::ADAPTER_MAP.delete(name)
+        ActiveModel::Serializer::Adapter.send(:remove_const, :AdapterFromEnvironment)
+      end
+
+      def test_get_adapter_for_unknown_name
+        assert_raises UnknownAdapterError do
+          ActiveModel::Serializer::Adapter.get(:json_simple)
+        end
+      end
+
+      def test_adapter
+        assert_equal ActiveModel::Serializer.config.adapter, :flatten_json
+        assert_equal ActiveModel::Serializer.adapter, ActiveModel::Serializer::Adapter::FlattenJson
+      end
+
+      def test_register_adapter
+        new_adapter_name  = :foo
+        new_adapter_klass = Class.new
+        ActiveModel::Serializer::Adapter.register(new_adapter_name, new_adapter_klass)
+        assert ActiveModel::Serializer::Adapter.adapters.include?("foo".freeze)
+        assert ActiveModel::Serializer::Adapter.get(:foo), new_adapter_klass
+      ensure
+        ActiveModel::Serializer::Adapter::ADAPTER_MAP.delete(new_adapter_name.to_s)
+      end
+
+      def test_inherited_adapter_hooks_register_adapter
+        Object.const_set(:MyAdapter, Class.new)
+        my_adapter = MyAdapter
+        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        assert_equal ActiveModel::Serializer::Adapter.get(:my_adapter), my_adapter
+      ensure
+        ActiveModel::Serializer::Adapter::ADAPTER_MAP.delete("my_adapter".freeze)
+        Object.send(:remove_const, :MyAdapter)
+      end
+
+      def test_inherited_adapter_hooks_register_demodulized_adapter
+        Object.const_set(:MyNamespace, Module.new)
+        MyNamespace.const_set(:MyAdapter, Class.new)
+        my_adapter = MyNamespace::MyAdapter
+        ActiveModel::Serializer::Adapter.inherited(my_adapter)
+        assert_equal ActiveModel::Serializer::Adapter.get(:my_adapter), my_adapter
+      ensure
+        ActiveModel::Serializer::Adapter::ADAPTER_MAP.delete("my_adapter".freeze)
+        MyNamespace.send(:remove_const, :MyAdapter)
+        Object.send(:remove_const, :MyNamespace)
+      end
     end
   end
 end

test/test_helper.rb

@@ -18,6 +18,12 @@
   @capture_warnings.after_tests
 end
 require 'active_model_serializers'
+# eager load autoloaded adapters
+require 'active_model/serializer/adapter'
+ActiveModel::Serializer::Adapter::Null
+ActiveModel::Serializer::Adapter::Json
+ActiveModel::Serializer::Adapter::FlattenJson
+ActiveModel::Serializer::Adapter::JsonApi
 
 # Use cleaner stream testing interface from Rails 5 if available
 # see https://github.com/rails/rails/blob/29959eb59d/activesupport/lib/active_support/testing/stream.rb