Added support for :only and :except methods.

It is possible now to filter returned attributes and associations by:

  UserSerializer.new(user, only: [:first_name, :last_name])
  UserSerializer.new(user, except: :first_name)

lib/active_model/serializer.rb

@@ -363,6 +363,8 @@ module ActiveModel
     end
 
     def include?(name)
+      return false if options.key?(:only) && !Array(options[:only]).include?(name)
+      return false if options.key?(:except) && Array(options[:except]).include?(name)
       send INCLUDE_METHODS[name]
     end
 

test/serializer_test.rb

@@ -1390,4 +1390,53 @@ class SerializerTest < ActiveModel::TestCase
     a_serializer = serializer.new(post, :scope => user, :scope_name => :current_user)
     assert a_serializer.has_permission?
   end
+
+  def test_only_option_filters_attributes_and_associations
+    post = Post.new(:title => "New Post", :body => "Body of new post")
+    comments = [Comment.new(:title => "Comment1")]
+    post.comments = comments
+
+    post_serializer = PostSerializer.new(post, :only => :title)
+
+    assert_equal({
+      :post => {
+        :title => "New Post"
+      }
+    }, post_serializer.as_json)
+  end
+
+  def test_except_option_filters_attributes_and_associations
+    post = Post.new(:title => "New Post", :body => "Body of new post")
+    comments = [Comment.new(:title => "Comment1")]
+    post.comments = comments
+
+    post_serializer = PostSerializer.new(post, :except => [:body, :comments])
+
+    assert_equal({
+      :post => {
+        :title => "New Post"
+      }
+    }, post_serializer.as_json)
+  end
+
+  def test_only_option_takes_precedence_over_custom_defined_include_methods
+    user = User.new
+
+    post = Post.new(:title => "New Post", :body => "Body of new post", :author => "Sausage King")
+    comments = [Comment.new(:title => "Comment")]
+    post.comments = comments
+
+    post_serializer = PostWithMultipleConditionalsSerializer.new(post, :scope => user, :only => :title)
+
+    # comments enabled
+    post.comments_disabled = false
+    # superuser - should see author
+    user.superuser = true
+
+    assert_equal({
+      :post => {
+        :title => "New Post"
+      }
+    }, post_serializer.as_json)
+  end
 end