Use respond_to?(:to_ary) rather than is_a?(Enumerable) to detect collection-thing.

activemodel/lib/active_model/serialization.rb

@@ -87,8 +87,8 @@ module ActiveModel
       Array(options[:methods]).each { |m| hash[m.to_s] = send(m) if respond_to?(m) }
 
       serializable_add_includes(options) do |association, records, opts|
-        hash[association.to_s] = if records.is_a?(Enumerable)
-          records.map { |a| a.serializable_hash(opts) }
+        hash[association.to_s] = if records.respond_to?(:to_ary)
+          records.to_ary.map { |a| a.serializable_hash(opts) }
         else
           records.serializable_hash(opts)
         end

activemodel/lib/active_model/serializers/xml.rb

@@ -115,7 +115,9 @@ module ActiveModel
           merged_options = opts.merge(options.slice(:builder, :indent))
           merged_options[:skip_instruct] = true
 
-          if records.is_a?(Enumerable)
+          if records.respond_to?(:to_ary)
+            records = records.to_ary
+
             tag  = ActiveSupport::XmlMini.rename_key(association.to_s, options)
             type = options[:skip_types] ? { } : {:type => "array"}
             association_name = association.to_s.singularize

activemodel/test/cases/serialization_test.rb

@@ -105,6 +105,24 @@ class SerializationTest < ActiveModel::TestCase
     assert_equal expected, @user.serializable_hash(:include => :friends)
   end
 
+  class FriendList
+    def initialize(friends)
+      @friends = friends
+    end
+
+    def to_ary
+      @friends
+    end
+  end
+
+  def test_include_option_with_ary
+    @user.friends = FriendList.new(@user.friends)
+    expected = {"email"=>"david@example.com", "gender"=>"male", "name"=>"David",
+                "friends"=>[{"name"=>'Joe', "email"=>'joe@example.com', "gender"=>'male'},
+                           {"name"=>'Sue', "email"=>'sue@example.com', "gender"=>'female'}]}
+    assert_equal expected, @user.serializable_hash(:include => :friends)
+  end
+
   def test_multiple_includes
     expected = {"email"=>"david@example.com", "gender"=>"male", "name"=>"David",
                 "address"=>{"street"=>"123 Lane", "city"=>"Springfield", "state"=>"CA", "zip"=>11111},

activemodel/test/cases/serializers/xml_serialization_test.rb

@@ -188,6 +188,23 @@ class XmlSerializationTest < ActiveModel::TestCase
     assert_match %r{<friend type="Contact">}, xml
   end
 
+  class FriendList
+    def initialize(friends)
+      @friends = friends
+    end
+
+    def to_ary
+      @friends
+    end
+  end
+
+  test "include option with ary" do
+    @contact.friends = FriendList.new(@contact.friends)
+    xml = @contact.to_xml :include => :friends, :indent => 0
+    assert_match %r{<friends type="array">}, xml
+    assert_match %r{<friend type="Contact">}, xml
+  end
+
   test "multiple includes" do
     xml = @contact.to_xml :indent => 0, :skip_instruct => true, :include => [ :address, :friends ]
     assert xml.include?(@contact.address.to_xml(:indent => 0, :skip_instruct => true))