diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index ef68072c..474b2d9d 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -134,12 +134,9 @@ module ActiveModel
             class_eval "def #{attr}() object.#{attr} end", __FILE__, __LINE__
           end
 
-          # if :key is specified without :serializer, then use conventions
-          # to determine the serializer
-          if options[:key] && !options[:serializer]
-            options[:serializer] = const_get("#{options[:key].to_s.camelize.singularize}Serializer")
-          else
-            options[:serializer] ||= const_get("#{attr.to_s.camelize}Serializer")
+          options[:serializer] ||= begin
+            serializer_class = (options[:key] || attr).to_s.classify
+            const_get("#{serializer_class}Serializer")
           end
 
           klass.new(attr, options)
diff --git a/test/serializer_test.rb b/test/serializer_test.rb
index 3a65e1ab..b4e50161 100644
--- a/test/serializer_test.rb
+++ b/test/serializer_test.rb
@@ -211,6 +211,30 @@ class SerializerTest < ActiveModel::TestCase
     }, json)
   end
 
+  def test_implicit_serializer_for_has_many
+    blog_with_posts = Class.new(Blog) do
+      attr_accessor :posts
+    end
+
+    blog_serializer = Class.new(ActiveModel::Serializer) do
+      const_set(:PostSerializer, PostSerializer)
+      has_many :posts
+    end
+
+    user = User.new
+    blog = blog_with_posts.new
+    blog.posts = [Post.new(:title => 'test')]
+
+    json = blog_serializer.new(blog, user).as_json
+    assert_equal({
+     :posts => [{
+       :title => "test", 
+       :body => nil, 
+       :comments => []
+     }]
+    }, json)
+  end
+
   def test_overridden_associations
     author_serializer = Class.new(ActiveModel::Serializer) do
       attributes :first_name