Work around the fact the JSON gem was overwriting to_json implementation for all Ruby core classes.

This is required because the JSON gem is incompatible with Rails behavior and was not allowing ActiveModel::Errors to be serialized.
So we need to ensure Rails implementation is the one triggered. [#4890 state:resolved]

activemodel/lib/active_model/errors.rb

@@ -3,6 +3,7 @@
 require 'active_support/core_ext/array/wrap'
 require 'active_support/core_ext/string/inflections'
 require 'active_support/core_ext/object/blank'
+require 'active_support/core_ext/hash/reverse_merge'
 require 'active_support/ordered_hash'
 
 module ActiveModel
@@ -164,15 +165,12 @@ def empty?
     #   #    <error>name must be specified</error>
     #   #  </errors>
     def to_xml(options={})
-      require 'builder' unless defined? ::Builder
-      options[:root]    ||= "errors"
-      options[:indent]  ||= 2
-      options[:builder] ||= ::Builder::XmlMarkup.new(:indent => options[:indent])
-
-      options[:builder].instruct! unless options.delete(:skip_instruct)
-      options[:builder].errors do |e|
-        to_a.each { |error| e.error(error) }
-      end
+      to_a.to_xml options.reverse_merge(:root => "errors", :skip_types => true)
+    end
+
+    # Returns an array as JSON representation for this object.
+    def as_json(options=nil)
+      to_a
     end
 
     # Adds +message+ to the error messages on +attribute+, which will be returned on a call to
@@ -283,7 +281,6 @@ def full_messages
     # <li><tt>errors.attributes.title.blank</tt></li>
     # <li><tt>errors.messages.blank</tt></li>
     # </ol>
-
     def generate_message(attribute, type = :invalid, options = {})
       type = options.delete(:message) if options[:message].is_a?(Symbol)
 

activemodel/test/cases/validations/presence_validation_test.rb

@@ -32,7 +32,7 @@ def test_validate_presences
     assert t.valid?
   end
 
-  test 'accepts array arguments' do
+  def test_accepts_array_arguments
     Topic.validates_presence_of %w(title content)
     t = Topic.new
     assert t.invalid?

activemodel/test/cases/validations_test.rb

@@ -6,6 +6,9 @@
 require 'models/custom_reader'
 require 'models/automobile'
 
+require 'active_support/json'
+require 'active_support/xml_mini'
+
 class ValidationsTest < ActiveModel::TestCase
 
   def setup
@@ -158,12 +161,18 @@ def test_invalid_validator
     end
   end
 
-  def test_errors_to_xml
-    r = Reply.new :title => "Wrong Create"
-    assert r.invalid?
-    xml = r.errors.to_xml(:skip_instruct => true)
-    assert_equal "<errors>", xml.first(8)
-    assert xml.include?("<error>Content is Empty</error>")
+  def test_errors_conversions
+    Topic.validates_presence_of %w(title content)
+    t = Topic.new
+    assert t.invalid?
+
+    xml = t.errors.to_xml
+    assert_match %r{<errors>}, xml
+    assert_match %r{<error>Title can't be blank</error>}, xml
+    assert_match %r{<error>Content can't be blank</error>}, xml
+
+    json = t.errors.to_json
+    assert_equal t.errors.to_a.to_json, json
   end
 
   def test_validation_order

activesupport/lib/active_support/json/encoding.rb

@@ -128,12 +128,21 @@ def escape(string)
   end
 end
 
-class Object
-  # Dumps object in JSON (JavaScript Object Notation). See www.json.org for more info.
-  def to_json(options = nil)
-    ActiveSupport::JSON.encode(self, options)
-  end
+# The JSON gem adds a few modules to Ruby core classes containing :to_json definition, overwriting
+# their default behavior. That said, we need to define the basic to_json method in all of them,
+# otherwise they will always use to_json gem implementation, which is backwards incompatible in
+# several cases (for instance, the JSON implementation for Hash does not work) with inheritance
+# and consequently classes as ActiveSupport::OrderedHash cannot be serialized to json.
+[Object, Array, FalseClass, Float, Hash, Integer, NilClass, String, TrueClass].each do |klass|
+  klass.class_eval <<-RUBY, __FILE__, __LINE__
+    # Dumps object in JSON (JavaScript Object Notation). See www.json.org for more info.
+    def to_json(options = nil)
+      ActiveSupport::JSON.encode(self, options)
+    end
+  RUBY
+end
 
+class Object
   def as_json(options = nil) #:nodoc:
     if respond_to?(:to_hash)
       to_hash

activesupport/test/ordered_hash_test.rb

 require 'abstract_unit'
+require 'active_support/json'
 
 class OrderedHashTest < Test::Unit::TestCase
   def setup
@@ -185,6 +186,12 @@ def test_inspect
     assert @ordered_hash.inspect.include?(@hash.inspect)
   end
 
+  def test_json
+    ordered_hash = ActiveSupport::OrderedHash[:foo, :bar]
+    hash = Hash[:foo, :bar]
+    assert_equal ordered_hash.to_json, hash.to_json
+  end
+
   def test_alternate_initialization_with_splat
     alternate = ActiveSupport::OrderedHash[1,2,3,4]
     assert_kind_of ActiveSupport::OrderedHash, alternate