Oj Rails Compatibility

The :rails mode mimics the ActiveSupport version 5 encoder. Rails and ActiveSupport are built around the use of the as_json(*) method defined for a class. Oj attempts to provide the same functionality by being a drop in replacement with a few exceptions.

require 'oj'


or simply call


Either of those steps will setup Oj to mimic Rails but it will not change the default mode type as the mode type is only used when calling the Oj encoding directly. If Rails mode is also desired then use the Oj.default_options to change the default mode.

Some of the Oj options are supported as arguments to the encoder if called from Oj::Rails.encode() but when using the Oj::Rails::Encoder class the encode() method does not support optional arguments as required by the ActiveSupport compliance guidelines. The general approach Rails takes for configuring encoding options is to either set global values or to create a new instance of the Encoder class and provide options in the initializer.

The globals that ActiveSupport uses for encoding are:

  • ActiveSupport::JSON::Encoding.use_standard_json_time_format

  • ActiveSupport::JSON::Encoding.escape_html_entities_in_json

  • ActiveSupport::JSON::Encoding.time_precision

  • ActiveSupport::JSON::Encoding.json_encoder

Those globals are aliased to also be accessed from the ActiveSupport module directly so ActiveSupport::JSON::Encoding.time_precision can also be accessed from ActiveSupport.time_precision. Oj makes use of these globals in mimicking Rails after the Oj::Rails.set_encode() method is called. That also sets the ActiveSupport.json_encoder to the Oj::Rails::Encoder class.

Options passed into a call to to_json() are passed to the as_json() methods. These are mostly ignored by Oj and simply passed on without modifications as per the guidelines. The exception to this are the options specific to Oj such as the :circular option which it used to detect circular references while encoding.

By default Oj acts like the ActiveSupport encoder and honors any changes in the as_json() methods. There are some optimized Oj encoders for some classes. When the optimized encoder it toggled the as_json() methods will not be called for that class but instead the optimized version will be called. The optimized version is the same as the ActiveSupport default encoding for a given class. The optimized versions are toggled with the optimize() and deoptimize() methods. There is a default optimized version for every class that takes the visible attributes and encodes them but that may not be the same as what Rails uses. Trial and error is the best approach for classes not listed here.

The classes that can be put in optimized mode and are optimized when Oj::Rails.optimize is called with no arguments are:

  • Array

  • BigDecimal

  • Float

  • Hash

  • Range

  • Regexp

  • Time

  • ActiveSupport::TimeWithZone

  • ActionController::Parameters

  • any class inheriting from ActiveRecord::Base

  • any other class where all attributes should be dumped

The ActiveSupport decoder is the JSON.parse() method. Calling the Oj::Rails.set_decoder() method replaces that method with the Oj equivalent.

Usage in Rails 3

To support Rails 3 you can create a new module mixin to prepend to controllers:

require 'oj'

module OjJsonEncoder
  def render(options = nil, extra_options = {}, &block)
    if options && options.is_a?(Hash) && options[:json]
      obj = options.delete(:json)
      obj = Oj.dump(obj, :mode => :rails) unless obj.is_a?(String)
      options[:text] = obj
      response.content_type ||= Mime::JSON


class MyController < ApplicationController
  prepend OjJsonEncoder
  def index
    render :json => { :hello => 'world' }

Older Ruby Version Support (Pre 2.3.0)

If you are using an older version of Ruby, you can pin oj to an earlier version in your Gemfile:

gem 'oj', '3.7.12'


  1. Optimized Floats set the significant digits to 16. This is different than Ruby which is used by the json gem and by Rails. Ruby varies the significant digits which can be either 16 or 17 depending on the value.

  2. Optimized Hashes do not collapse keys that become the same in the output. As an example, a non-String object that has a to_s() method will become the return value of the to_s() method in the output without checking to see if that has already been used. This could occur is a mix of String and Symbols are used as keys or if a other non-String objects such as Numerics are mixed with numbers as Strings.

  3. To verify Oj is being used turn on the Oj :trace option. Similar to the Ruby Tracer Oj will then print out trace information. Another approach is to turn on C extension tracing. Set tracer = TracePoint.new(:c_call) do |tp| p [tp.lineno, tp.event, tp.defined_class, tp.method_id] end or, in older Rubies, set Tracer.display_c_call = true.

For example:

 require 'active_support/core_ext'
 require 'active_support/json'
 require 'oj'
 tracer.enable { Time.now.to_json }
 # prints output including
 [20, :c_call, #<Class:Oj::Rails::Encoder>, :new]
 [20, :c_call, Oj::Rails::Encoder, :encode]
 => "\"2018-02-23T12:13:42.493-06:00\""