Improved compatibility with the stdlib JSON gem.

Previously, calling `::JSON.{generate,dump}` sometimes causes
unexpected failures such as intridea/multi_json#86.

`::JSON.{generate,dump}` now bypasses the ActiveSupport JSON encoder
completely and yields the same result with or without ActiveSupport.
This means that it will **not** call `as_json` and will ignore any
options that the JSON gem does not natively understand. To invoke
ActiveSupport's JSON encoder instead, use `obj.to_json(options)` or
`ActiveSupport::JSON.encode(obj, options)`.

1 files changed, 45 insertions, 0 deletions

diff --git a/activesupport/test/json/encoding_test.rb b/activesupport/test/json/encoding_test.rb
index 856ca75cbc..39da760bf2 100644
--- a/activesupport/test/json/encoding_test.rb
+++ b/activesupport/test/json/encoding_test.rb
@@ -32,6 +32,19 @@ class TestJSONEncoding < ActiveSupport::TestCase
     end
   end
 
+  class HashWithAsJson < Hash
+    attr_accessor :as_json_called
+
+    def initialize(*)
+      super
+    end
+
+    def as_json(options={})
+      @as_json_called = true
+      super
+    end
+  end
+
   TrueTests     = [[ true,  %(true)  ]]
   FalseTests    = [[ false, %(false) ]]
   NilTests      = [[ nil,   %(null)  ]]
@@ -367,6 +380,38 @@ class TestJSONEncoding < ActiveSupport::TestCase
     assert_equal false, false.as_json
   end
 
+  def test_json_gem_dump_by_passing_active_support_encoder
+    h = HashWithAsJson.new
+    h[:foo] = "hello"
+    h[:bar] = "world"
+
+    assert_equal %({"foo":"hello","bar":"world"}), JSON.dump(h)
+    assert_nil h.as_json_called
+  end
+
+  def test_json_gem_generate_by_passing_active_support_encoder
+    h = HashWithAsJson.new
+    h[:foo] = "hello"
+    h[:bar] = "world"
+
+    assert_equal %({"foo":"hello","bar":"world"}), JSON.generate(h)
+    assert_nil h.as_json_called
+  end
+
+  def test_json_gem_pretty_generate_by_passing_active_support_encoder
+    h = HashWithAsJson.new
+    h[:foo] = "hello"
+    h[:bar] = "world"
+
+    assert_equal <<EXPECTED.chomp, JSON.pretty_generate(h)
+{
+  "foo": "hello",
+  "bar": "world"
+}
+EXPECTED
+    assert_nil h.as_json_called
+  end
+
   protected
 
     def object_keys(json_object)