Question

我使用Cancan 1.6.9进行Rails 3应用程序授权。我想将Ability类转换为JSON。这是我的能力课程。

# 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

当我运行Ability.for(User.last).to_json时，我收到以下错误：

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/active_support/json/encoding.rb:46:in `encode'
    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:246:in `block in encode_json'
    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:246:in `each'
    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:246:in `map'
    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:246:in `encode_json'
    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:48:in `block in encode'
    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:77: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/active_support/json/encoding.rb:46:in `encode'
    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:246:in `block in encode_json'
    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:246:in `each'
    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:246:in `map'
    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:246:in `encode_json'
    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:48:in `block in encode'
    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:77: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/active_support/json/encoding.rb:46:in `encode'
... 19 levels...
    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:46:in `encode'
    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:246:in `block in encode_json'
    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:246:in `each'
    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:246:in `map'
    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:246:in `encode_json'
    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:48:in `block in encode'
    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:77: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/active_support/json/encoding.rb:46:in `encode'
    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:31:in `encode'
    from /home/user/.rvm/gems/ruby-1.9.3-p392@rails-3.2.13/gems/activesupport-3.2.13/lib/active_support/core_ext/object/to_json.rb:16:in `to_json'
    enter code here

当我跑Ability.for(User.last).as_json时，我得到了结果。但它不是JSON的有效结果。

{"rules"=>[#<CanCan::Rule:0xaf5e81c @match_all=false, @base_behavior=false, @actions=[:manage], @subjects=[User(id: integer, password_digest: string, created_at: datetime, updated_at: datetime, uic: string, role: string, name: string, register_no: string)], @conditions={}, @block=nil>, #<CanCan::Rule:0xaf5e72c @match_all=false, @base_behavior=true, @actions=[:read], @subjects=[ExchangeRate(id: integer, data: float, date: date, created_at: datetime, updated_at: datetime)], @conditions={}, @block=nil>]}

有什么想法吗？

Answer 1

TL; DR：在模型中实施self.as_json方法，并明确指定要转换为JSON的字段。在此之后，您将能够毫无错误地运行Ability.for(User.last).to_json。

完整说明：让我们看一下从source看到的Ability的结构：

Ability实例有一个名为@rules。
每个规则都有一个名为@subject的字段，它是一个展平的ActiveRecord类（User，ExchangeRate等）。是的， class ，而不是类实例。
@subject转换为JSON并显示CircularReferenceError。为什么？

问题是，将User或ExchangeRate类转换为JSON并不是一个好主意，因为它们的结构可能非常复杂。例如，如果您尝试运行User.as_json，结果可能类似于this。可能User个字段中的一个引用回User，您得到CircularReferenceError。要避免这种情况，请指定哪些字段应包含在User的JSON表示中（同样适用于ExchangeRate）：

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

Cancan：json的能力等级

1 个答案: