# 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.

```ruby
require 'oj'

Oj::Rails.set_encoder()
Oj::Rails.set_decoder()
Oj::Rails.optimize()
Oj::Rails.mimic_JSON()
```

or simply call

```ruby
Oj.optimize_rails()
```

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 mimicing
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:

```ruby
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
    end
    super
  end
end
```

Usage:

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

### 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:

```ruby
gem 'oj', '3.7.12'
```

### Notes:

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 Hashs 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'
     Oj.optimize_rails
     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\""
     ```