Three - an even smaller, tinier simple authorization gem for ruby

This gem started as a minor fork of six, a neat, tiny authorization gem. I used six and liked it, but as small was it was I found that I needed maybe half of its code and features. So here is three.

three is a small authentication library, focused on only a few needs:

1. Provide an open/closed method of constructing rules,
2. Provide a way to remove permissions, and
3. Do it as simply as possible.

Installation

  gem install three

Usage

Here's the simplest working example... a set of rules that apply for all

# make an object with an "allowed" method that returns an array of permissions
module Rules
  def self.allowed _, _
    [:edit, :delete]
  end
end

# create an evaluator that can be used to evaluate rules
evaluator = Three.evaluator_for Rules

# use the evaluator to determine what's allowed or not

evaluator.allowed? nil, :edit   # true
evaluator.allowed? nil, :close  # false
evaluator.allowed? nil, :delete # true

Unfortunately, that's not a very realistic example. We'll almost always want to evaluate the rules based on some sort of subject:

module AdminRules
  def self.allowed user, _
    return [] unless user.admin?
    [:edit, :delete]
  end
end

evaluator = Three.evaluator_for AdminRules

admin_user   = User.new(admin: true)
not_an_admin = User.new(admin: false)

evaluator.allowed? admin_user, :edit        # true
evaluator.allowed? not_an_admin_user, :edit # false

See? The array of permissions returned by the "allowed" method are used to determine if a user can do something.

The rules can be compounded, like so:

module AdminRules
  def self.allowed user, _
    return [] unless user.admin?
    [:edit, :delete]
  end
end

module UserRules
  def self.allowed user, _
    return [] if user.admin?
    [:view_my_account]
  end
end

evaluator = Three.evaluator_for(AdminRules, UserRules)

admin_user   = User.new(admin: true)
not_an_admin = User.new(admin: false)

evaluator.allowed? admin_user, :edit        # true
evaluator.allowed? not_an_admin_user, :edit # false

evaluator.allowed? admin_user, :view_my_account        # false
evaluator.allowed? not_an_admin_user, :view_my_account # true

But what about that trailing "_" variable? That's used as an optional target, which you can use to return permissions based on the relationship between the two arguments:

module MovieRules
  def self.allowed user, movie
    if user.is_a_minor? and movie.is_rated_r
      []
    else
      [:can_buy_the_ticket]
    end
  end
end

evaluator = Three.evaluator_for MovieRules

minor       = User.new(minor: true)
not_a_minor = User.new(minor: false)

scary_movie = Movie.new(rating: 'R')
kids_movie  = Movie.new(rating: 'PG')

evaluator.allowed? minor, :can_buy_the_ticket, scary_movie        # false
evaluator.allowed? not_a_minor, :can_buy_the_ticket, scary_movie  # true

evaluator.allowed? minor, :can_buy_the_ticket, kids_movie        # true
evaluator.allowed? not_a_minor, :can_buy_the_ticket, kids_movie  # true

Only one more special thing... what if we want to right a rule that prevents something?

module DefaultLibraryRules
  def self.allowed user, book
    [:reserve_the_book]
  end
end

module FinesOwedRules
  def self.prevented user, _
    if user.owes_fines?
      [:reserve_the_book]
    else
      []
    end
  end
end

evaluator = Three.evaluator_for(DefaultLibraryRules, FinesOwedRules)

deadbeat            = User.new(fines: 3.0)
responsible_citizen = User.new(fines: 0)

evaluator.allowed? deadbeat, :reserve_the_book             # false
evaluator.allowed? responsible_citizen, :reserve_the_book  # true

The "prevented" method works just like "allowed," except that it will remove the permission from any other rule's "allowed" method.

The "prevented" method is the only only feature added with six.

Errors

By default, errors in a rule or calling a rule is turned off. This means that you don't have to declare "allowed" or "prevented" on your rules, and you can have clean examples like the one above.

However, sometimes you may not want to run your production code through blanket rescue statements. So, you can disable this using:

module RulesMissingMethods
end

evaluator = Three.evaluator_for RulesMissingMethods
evaluator.rescue_errors = false

evaluator.allowed? nil, :watch_out # POW an error was raised

Tracing

Ok, so if your security rights are broken out into many different classes, it might be helpful to which one is allowing or preventing permissions.

If you'd like to take a peek behind the curtain, try the following:

Three.when_tracing do |what, details|
  # "what" will be :allowed/:prevented
  #   details is a hash with the following:
  #   subject     # the subject of the rules check
  #   target      # the target, if one was provided
  #   permissions # the permissions either allowed or prevented
  #   rule        # the rule making the check
  puts [what, details].inspect
end