Custom JSON types in AS 4.1+

gistfile1.rb

# config/initializers/lol_json.rb

module ActiveSupport
  module JSON
    module Encoding
      class JSONGemEncoder
        BYPASS_JSONIFY = Set.new
        
        alias_method :original_jsonify, :jsonify
        
        def jsonify(value)
          BYPASS_JSONIFY.include?(value.class) ? value : original_jsonify(value)
        end
      end
    end
  end
end

# lib/compiled_json.rb

class CompiledJson
  def initialize(s); @s = s; end
  def to_json(*); @s; end
  def as_json(*); self; end
end

ActiveSupport::JSON::Encoding::JSONGemEncoder::BYPASS_JSONIFY << CompiledJson

# seems to be working

bar = CompiledJson.new("bar")

MultiJson.dump(foo: bar)               # => { "foo": bar }
{ foo: bar }.to_json                   # => { "foo": bar }
Jbuilder.encode{ |json| json.foo bar } # => { "foo": bar }