Require explicit adapter/serializer to render JSON API errors

- Separate collection errors from resource errors in adapter
- Refactor to ErrorsSerializer; first-class json error methods
- DOCS
- Rails 4.0 requires assert exact exception class, boo

lib/active_model/serializable_resource.rb

@@ -18,14 +18,6 @@ module ActiveModel
         options.partition { |k, _| ADAPTER_OPTION_KEYS.include? k }.map { |h| Hash[h] }
     end
 
-    def errors?
-      if resource.respond_to?(:each)
-        resource.any? { |elem| elem.respond_to?(:errors) && !elem.errors.empty? }
-      else
-        resource.respond_to?(:errors) && !resource.errors.empty?
-      end
-    end
-
     def serialization_scope=(scope)
       serializer_opts[:scope] = scope
     end
@@ -39,11 +31,7 @@ module ActiveModel
     end
 
     def adapter
-      @adapter ||=
-        begin
-          adapter_opts[:adapter] = :'json_api/error' if errors?
-          ActiveModelSerializers::Adapter.create(serializer_instance, adapter_opts)
-        end
+      @adapter ||= ActiveModelSerializers::Adapter.create(serializer_instance, adapter_opts)
     end
     alias_method :adapter_instance, :adapter
 
@@ -58,7 +46,6 @@ module ActiveModel
       @serializer ||=
         begin
           @serializer = serializer_opts.delete(:serializer)
-          @serializer = ActiveModel::Serializer::ErrorSerializer if errors?
           @serializer ||= ActiveModel::Serializer.serializer_for(resource)
 
           if serializer_opts.key?(:each_serializer)

lib/active_model/serializer.rb

@@ -2,6 +2,7 @@ require 'thread_safe'
 require 'active_model/serializer/collection_serializer'
 require 'active_model/serializer/array_serializer'
 require 'active_model/serializer/error_serializer'
+require 'active_model/serializer/errors_serializer'
 require 'active_model/serializer/include_tree'
 require 'active_model/serializer/associations'
 require 'active_model/serializer/attributes'

lib/active_model/serializer/error_serializer.rb

@@ -1,2 +1,6 @@
 class ActiveModel::Serializer::ErrorSerializer < ActiveModel::Serializer
+  # @return [Hash<field_name,Array<error_message>>]
+  def as_json
+    object.errors.messages
+  end
 end

lib/active_model/serializer/errors_serializer.rb

@@ -0,0 +1,23 @@
+require 'active_model/serializer/error_serializer'
+class ActiveModel::Serializer::ErrorsSerializer < ActiveModel::Serializer
+  include Enumerable
+  delegate :each, to: :@serializers
+  attr_reader :object, :root
+
+  def initialize(resources, options = {})
+    @root = options[:root]
+    @object = resources
+    @serializers = resources.map do |resource|
+      serializer_class = options.fetch(:serializer) { ActiveModel::Serializer::ErrorSerializer }
+      serializer_class.new(resource, options.except(:serializer))
+    end
+  end
+
+  def json_key
+    nil
+  end
+
+  protected
+
+  attr_reader :serializers
+end