Cancan: Ability class to json

Question

I have Rails 3 application with Cancan 1.6.9 for authorization. I want to convert Ability class to JSON. Here is the my Ability class. ``` # encoding: utf-8 module Ability def self.for(guest) guest ||= User.new # guest user (not logged in) if guest.admin? AdminAbility.new(guest) elsif guest.assurer? AssurerAbility.new(guest) end end ## # Assurer class AssurerAbility include CanCan::Ability def initialize(user) cannot :manage, User can :read, ExchangeRate end end ## # Admin class AdminAbility include CanCan::Ability def initialize(user) can :manage, User can :manage, ExchangeRate end end end ``` When I run `Ability.for(User.last).to_json` I get following errors: ``` ActiveSupport::JSON::Encoding::CircularReferenceError: object references itself from /home/user/.rvm/gems/ruby-1.9.3-p392@rails-3.2.13/gems/activesupport-3.2.13/lib/active_support/json/encoding.rb:75:in `check_for_circular_references' from /home/user/.rvm/gems/ruby-1.9.3-p392@rails-3.2.13/gems/activesupport-3.2.13/lib/activ…

Accepted Answer

TL;DR: Implement `self.as_json` method in your models and explicitly specify which fields to convert to JSON. After this you will be able to run `Ability.for(User.last).to_json` without errors.

Complete explanation: let's look at the structure of `Ability` as seen from source:

- `Ability` instance has a field named `@rules`.

- Each rule has a field named `@subject` which is a flattened ActiveRecord class (`User`, `ExchangeRate` etc). Yes, class, not class instance.

- `@subject` gets converted to JSON and `CircularReferenceError` appears. Why?

The problem is, converting `User` or `ExchangeRate` class to JSON is not a good idea because their structure may be very complicated. For instance, if you try running `User.as_json`, the result might be like this. Probably one of `User`'s fields references back to `User` and you get `CircularReferenceError`. To avoid this, specify which fields should be included into JSON representation of `User` (the same is applied for `ExchangeRate`):

```
class User < ActiveRecord::Base
  #...
  def self.as_json(options = {})
    { "class" => "User" }
  end
end
```

Cancan: Ability class to json

Solution

Problem