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, 21 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/core_ext/object/json.rb b/activesupport/lib/active_support/core_ext/object/json.rb
index 898c3f4307..5157b0402f 100644
--- a/activesupport/lib/active_support/core_ext/object/json.rb
+++ b/activesupport/lib/active_support/core_ext/object/json.rb
@@ -9,18 +9,36 @@ require 'time'
 require 'active_support/core_ext/time/conversions'
 require 'active_support/core_ext/date_time/conversions'
 require 'active_support/core_ext/date/conversions'
+require 'active_support/core_ext/module/aliasing'
 
 # 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.
+# 
+# On the other hand, we should avoid conflict with ::JSON.{generate,dump}(obj). Unfortunately, the
+# JSON gem's encoder relies on its own to_json implementation to encode objects. Since it always
+# passes a ::JSON::State object as the only argument to to_json, we can detect that and forward the
+# calls to the original to_json method.
+# 
+# It should be noted that when using ::JSON.{generate,dump} directly, ActiveSupport's encoder is
+# bypassed completely. This means that as_json won't be invoked and the JSON gem will simply
+# ignore any options it does not natively understand. This also means that ::JSON.{generate,dump}
+# should give exactly the same results with or without active support.
 [Object, Array, FalseClass, Float, Hash, Integer, NilClass, String, TrueClass].each do |klass|
   klass.class_eval do
-    # Dumps object in JSON (JavaScript Object Notation). See www.json.org for more info.
-    def to_json(options = nil)
-      ActiveSupport::JSON.encode(self, options)
+    def to_json_with_active_support_encoder(options = nil)
+      if options.is_a?(::JSON::State)
+        # Called from JSON.{generate,dump}, forward it to JSON gem's to_json
+        self.to_json_without_active_support_encoder(options)
+      else
+        # to_json is being invoked directly, use ActiveSupport's encoder
+        ActiveSupport::JSON.encode(self, options)
+      end
     end
+
+    alias_method_chain :to_json, :active_support_encoder
   end
 end