Scram

An awesome authorization system

Installation

Add this line to your application's Gemfile:

gem 'scram'

And then execute:

$ bundle

Or install it yourself as:

$ gem install scram

Usage

Click here to see YARD Documentation

Quick Overview of Scram

• Holder
  • Scram doesn't force you to use a specific group system. Instead, just include Holder into any class which can hold Policies.
  • Scram provides a class for objects like Groups through an AggregateHolder. This is a class which should be included in anything which holds policies through other holders.
  • In most cases, your AggregateHolder will be a User model. Your Group model will be a Holder. If you don't want to use a group system, then your User model will likely be a Holder.
• Policy
  • Policies are used to bundle together permissions.
  • There are 2 kinds of Policies: Those for a specific model, and "global" Policies for permissions that aren't bound to a specific model.
• Target
  • Targets are a way to declare what actions are allowed in a Policy.
  • Targets have a list of actions and conditions.
    • Actions are anything a user can do to an object. For example: :update, :view, :create, :destroy.
    • Conditions are used to refine what instances a target applies to. They support basic comparisons to attributes, but can be used to support more complex logic with the DSL.

Example Usage

If you choose to implement Holder into your user class directly, it may look something like the following.

class User
  ...
  include Scram::Holder

  # Will automatically implement the needed #policies method for Holder.
  has_many :policies, class: "Scram::Policy"

  def scram_compare_value
    self.id
  end
end

This sort of system would not include a group system at all (for simplicity). If you want a Group system, have your user include Scram::AggregateHolder and then implement #aggregates to return your groups (which should be Holders themselves).

We will be providing a full fledge example application of Scram shortly which will include a Group and membership system, and will clarify how the AggregateHolder system works. For now, lets see how Scram works in its simplest usage (a user who stores policies just for themselves).

Adding a String Permission

Now lets add a String permission to display a statistics bar for users like admins. We want to call user.can? :view, "peek_bar" and have it return true for admins.

To do this, we'll need to define a non-model Policy (because our object is a string, "peek_bar").

user = ...
policy = Scram::Policy.new
policy.name = "global-strings-policy" # Note that we're setting name, and we will leave context nil.
policy.context = nil # This would be nil by default as well. By not setting this to anything, we let this Policy handle String permissions, and not be bound to a model.
policy.save
user.policies << policy
user.save

Now we want to add a target that represents the ability to view "peek_bar".

target = Target.new
target.conditions = {:equals => { :'*target_name' =>  "peek_bar"}}
target.actions << "view"
policy.targets << target
policy.save

This code creates a target which permits viewing if the *target_name equals "peek_bar".

Scram automatically replaces *target_name with the action being compared. For example, in can? :view, "something_else" Scram would check if "something_else" == "peek_bar".

And now we're done! 🎉

Adding a Model Permission

Now lets add something a bit more complex. Say we're developing a Forum application. We want to add the ability for a user to edit their own Posts using Scram.

Here's our simple Post model:

class Post
  ...
  belongs_to :user
end

Lets make a Policy that handles post related permissions.

user = ...
policy = Scram::Policy.new
policy.name = "Post Stuff" # This name is just for organizational/display purposes
policy.context = Post.to_s # Note: By setting context, we bind this policy to the model "Post"
policy.save
user.policies << policy
user.save

Now we need a Target in our Policy to let users edit their own Posts.

target = Target.new
target.conditions = {:equals => {:user => "*holder"}}
target.actions << "edit"
policy.save

What is *holder? Well, Scram replaces this special variable with the current user being compared. In User#scram_compare_value we return the User's ObjectId, and this is exactly what Scram replaced *holder with.

So now this Target reads "allow a holder to edit a Post if the user of that Post is the holder". Pretty neat, huh?

And now we're done! Go ahead and call holder.can? :edit, @post on a post which they own, and you'll see that Scram allows it! 🎉

Using conditions

In our last example, we let Scram directly compare an attribute of the model. What if we need more complex checking behavior? Luckily, Scram provides a DSL for models to easily define conditions which can be referenced in the database in place of attributes.

Lets revisit the Post example, but this time we'll define how to get the owner using a condition DSL, instead of the attribute.

class Post
  include Scram::DSL::ModelConditions
  ...
  belongs_to :user

  scram_define do
    condition :owner do |post|
      post.user
    end
  end
end

Now we no longer need to directly tell our Target to access the user field. Here's what the equivalent Target would look like from our previous example, now using the new condition:

...
target.conditions = {:equals => {:'*owner' => "*holder"}}
...

Scram is smart enough to realize that any key starting with an *, like *owner, is a manually defined condition. Now, calling user.can? :edit, @post will compare the value returned by the condition block to the hash value (which in this case is the Holder).

Defining a New Comparator

You may have noticed from the previous examples that the keys of our Target conditions were things like equals and less_than. These come from our Comparator definitions (see Scram::DSL::Definitions::COMPARATORS).

These comparators are defined using the DSL for comparators. We provide a basic set of comparing operators, but you may need to add your own. To do this, we recommend creating an initializer file and then calling something like the following:

builder = Scram::DSL::Builders::ComparatorBuilder.new do
  comparator :asdf do |a,b|
    true
  end
end
Scram::DSL::Definitions.add_comparators(builder)

Now your targets can use asdf as a conditions key, and Scram will use your method of comparison to determine if something is true or not. In this case, asdf returns true regardless of the two objects being compared.

Gotchas

Having trouble trying to use a holder check on a relation? Easy fix! The issue you are experiencing is just that the holder's scram_compare_value will probably be an ObjectId of some sort, but if you are comparing it against the relation... you are trying to compare the current holder's ObjectId to a document. The fix to this is just defining a condition within the model with the user you are trying to compare, and returning the object id of that.

Example of the issue: Lets say your Post model belongs_to :user. If you tried setting up a condition which checks something like this: :equals => { :'user' => "*holder" } it will never work because of the above description. To fix it, define a condition which returns an ObjectId.

scram_define do
  condition :owner do |post|
    post.user.scram_compare_value # we could also have done post.user.id
  end
end

Now update your Target to have the following condition: :equals => { :'*owner' => "*holder" }. Voila! It will all work now, because you are correctly comparing the right data types.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/skreem/scram.