What is authorization?
Authorization is a process in which we allow or restrict access to certain resources.
Let's assume that task-1 is assigned to Oliver and task-2 is assigned to Eve. The way the code is right now, Oliver can see Eve's tasks and Eve can see Oliver's task. That's not right.
So, let us introduce authorization to our application.
Features
By the end of the chapter, our application will have the following changes.
- Only the tasks assigned to, or created by the logged in user, will be displayed in their dashboard.
- Only the creator or assignee will be allowed to view the details of a
Task. For others, the request will result in authorization error. - Only the creator of a task will be allowed to delete it.
- Only the creator is allowed to change title or to re-assign the task.
Technical design
We will be using Pundit gem to do authorization work.
Pundit helps us in managing role based authorization using policies defined in simple Ruby Classes.
- We will create a new Ruby class named
TaskPolicyand define the authorization conditions in it. - Pundit associates methods in this class with our controller actions. It executes the method corresponding to the action when a request is received. If its result is false, access is denied and an error is thrown. Otherwise the action will be executed as normal.
- This approach don't work for our index action. We need it to run for every authenticated user. The requirement is to narrow down the results to only the ones that are relevant to the user.
- We can use the policy scopes of pundit to filter the tasks. It will let us add additional conditions on the method we use to fetch tasks from database. Therefore the result will contain only those tasks that the requesting user is authorized to see.
Let us see this in action.
Pundit gem installation
Add the this line to the Gemfile:
1gem "pundit"Now, install the gem:
1bundle installInclude Pundit in the application controller:
1class ApplicationController < ActionController::Base
2 include Pundit
3endIntroducing policies
Policies contain the authorization for an action. Let's create a policy:
1mkdir app/policies
2touch app/policies/task_policy.rbOpen task_policy.rb and add following lines of code:
1class TaskPolicy
2 attr_reader :user, :task
3
4 def initialize(user, task)
5 @user = user
6 @task = task
7 end
8
9 def show?
10 # some condition which returns a boolean value
11 end
12endPundit makes the following assumptions about this class:
- The class has the same name as that of model class, only suffixed with the word "Policy". Hence, the name TaskPolicy.
- The first argument is a
user. In your controller, Pundit will call thecurrent_usermethod, which we had defined inApplicationController, to retrieve what to send into this argument. - The second argument is that of a model object, whose authorization you want to check.
- The class implements some kind of query method, in this case
show?. Usually, this will map to the name of a particular controller action.
Adding policy check in TaskPolicy
Now let's look at the required code for class TaskPolicy:
1class TaskPolicy
2 attr_reader :user, :task
3
4 def initialize(user, task)
5 @user = user
6 @task = task
7 end
8
9 # The show policy check is invoked when we call `authorize @task`
10 # from the show action of tasks controller.
11 # Here the condition we want to check is that
12 # whether the record's creator is current user or record is assigned to the current user.
13 def show?
14 task.task_owner_id == user.id || task.assigned_user_id == user.id
15 end
16
17 # The condition for edit policy is the same as that of the show.
18 # Hence, we can simply call `show?` inside the edit? policy here.
19 def edit?
20 show?
21 end
22
23 # Only owner is allowed to update a task.
24 def update?
25 task.task_owner_id == user.id
26 end
27
28 # Every user can create a task, hence create? will always returns true.
29 def create?
30 true
31 end
32
33 # Only the user that has created the task, can delete it.
34 def destroy?
35 task.task_owner_id == user.id
36 end
37endNow, let's add the authorize method to our controller actions. The required code is added as follows:
1class TasksController < ApplicationController
2 before_action :authenticate_user_using_x_auth_token
3 before_action :load_task, only: %i[show update destroy]
4
5 def index
6 tasks = Task.all.as_json(include: { assigned_user: { only: %i[name id] } })
7 render status: :ok, json: { tasks: tasks }
8 end
9
10 def create
11 task = current_user.created_tasks.new(task_params)
12 authorize task
13 if task.save
14 render status: :ok,
15 json: { notice: t('successfully_created', entity: 'Task') }
16 else
17 errors = task.errors.full_messages.to_sentence
18 render status: :unprocessable_entity, json: { error: errors }
19 end
20 end
21
22 def show
23 authorize @task
24 end
25
26 def update
27 authorize @task
28 if @task.update(task_params)
29 render status: :ok, json: {}
30 else
31 render status: :unprocessable_entity,
32 json: { error: @task.errors.full_messages.to_sentence }
33 end
34 end
35
36 def destroy
37 authorize @task
38 if @task.destroy
39 render status: :ok, json: {}
40 else
41 render status: :unprocessable_entity,
42 json: { error: @task.errors.full_messages.to_sentence }
43 end
44 end
45
46 private
47
48 def task_params
49 params.require(:task).permit(:title, :assigned_user_id)
50 end
51
52 def load_task
53 @task = Task.find_by(slug: params[:slug])
54 unless @task
55 render status: :not_found, json: { error: t('task.not_found') }
56 end
57 end
58endLet's define authorization.denied message in our en.yml file to access it in
the update action:
1en:
2 authorization:
3 denied: "Access denied. You are not authorized to perform this action."
4 session:
5 could_not_auth: "Could not authenticate with the provided credentials."
6 incorrect_credentials: "Incorrect credentials, try again."
7 successfully_created: "%{entity} was successfully created!"And in our app/javascript/src/components/Tasks/EditTask.jsx file, whenever we
are making the API call, send the authorization_owner param as true, so that
in backend we can verify whether the one who is modifying the task is the owner
itself or not:
1// --------previous code --------
2
3const handleSubmit = async event => {
4 event.preventDefault();
5 try {
6 await tasksApi.update({
7 slug,
8 payload: {
9 task: { title, assigned_user_id: userId }
10 }
11 });
12 setLoading(false);
13 history.push("/");
14 } catch (error) {
15 setLoading(false);
16 logger.error(error);
17 }
18};We have added the line authorize @task to show, update and destroy actions
after initializing our @task instance variable.
The authorize method automatically infers that Task will have a matching
TaskPolicy class, and instantiates this class, handing in the current user and
the given record (@task in this case).
It then infers from the action name, that it should call the respective action
of class TaskPolicy.
For example, the instance created by the authorize @task inside the show action,
will call the show? action of Policy class.
From a high level overview, pundit authorize method in TasksController#show
action, works something similar to below mentioned code:
1unless TaskPolicy.new(current_user, @task).show?
2 raise Pundit::NotAuthorizedError, "not allowed to show? this #{@task.inspect}"
3endThis raises an exception. But Pundit allows us to rescue the exception with a
method of our choice. So, let's add the code for that in our
application_controller.rb:
1class ApplicationController < ActionController::Base
2 protect_from_forgery with: :exception
3 include Pundit
4 rescue_from Pundit::NotAuthorizedError, with: :handle_authorization_error
5#------previous code -------
6
7 private
8
9 #------previous code -------
10 def handle_authorization_error
11 render status: :forbidden, json: { error: t("authorization.denied") }
12 end
13endSo whenever an action in TaskPolicy returns false it means the authorization has
failed. When an authorization fails then we are raising an exception. The
exception is rescued by method handle_authorization_error.
Let's say there exists a task with slug of task-slug and a user with name
"Oliver". Task with slug of task-slug is neither created by Oliver nor
assigned to Oliver. Oliver logs in by entering his email and password. Now
Oliver, enters the URL localhost:3000/tasks/task-slug/show.
Since Oliver is not authorized to view the task because of the above
assumptions, rather than throwing him an error (the red page), Pundit raises an
exception and rescues it by calling the method handle_authorization_error.
Introducing policy scope
If you closely observe, we did not make a change to our index action. As index
action returns a collection of records, we need to apply a condition on a
collection, and we do that by using Policy Scope.
As we want to have our index view to display only the tasks which are either
created by the current_user or assigned to the current_user, we will define a
class called a policy scope. This class is nested inside the class TaskPolicy:
1class TaskPolicy
2
3 #------previous code -------
4
5 #------add new lines here----
6 class Scope
7 attr_reader :user, :scope
8
9 def initialize(user, scope)
10 @user = user
11 @scope = scope
12 end
13
14 def resolve
15 scope.where(task_owner_id: user.id).or(scope.where(assigned_user_id: user.id))
16 end
17 end
18endPundit makes the following assumptions about this class:
The class has the name Scope and is nested under the policy class.
The first argument is a user. In your controller, Pundit will call the current_user method to retrieve what to send into this argument.
The second argument is a scope which is a collection of records.
Instances of this class respond to the method resolve.
This method contains the query run on the scope defined and then returns a result which is a collection and can be iterated over.
The corresponding change we make in the index action of Tasks controller is as follows:
1def index
2 #------new line added here------
3 tasks = TaskPolicy::Scope.new(current_user, Task).resolve
4 #-----end of added line-------
5endLet's observe what's going on here-
Pundit creates an instance of class Scope (nested inside the class TaskPolicy) passing along the
current_userand theTaskmodel (ourscopein this case) as parameters which get set in the@userand@scopeinstance variables respectively inside the initialize method.Now this instance calls the method
resolvewhere we run a query on our scope and it returns a collection of tasks which only have the tasks that are either created by or assigned to the current_user.
Now to make it easier Pundit provides syntactic sugar where we can replace the
line TaskPolicy::Scope.new(current_user, Task).resolve simply with the syntax
policy_scope(Task). It works exactly the same way as described before.
Now, replace your index method with the following lines:
1def index
2 tasks = policy_scope(Task)
3 tasks_with_assigned_user = tasks.as_json(include: { assigned_user: { only: %i[name id] } })
4 render status: :ok, json: { tasks: tasks_with_assigned_user }
5endNow, we have added authorization to all our actions in task controller. But what if we forgot to call authorize function in every action? Or if someone changed the code and removed it by accident?
It would be a serious security loophole.
To catch such errors in a fail-fast way, Pundit provides us some helper
methods.
Add these lines to our TasksController:
1class TasksController < ApplicationController
2 after_action :verify_authorized, except: :index
3 after_action :verify_policy_scoped, only: :index
4 before_action :authenticate_user_using_x_auth_token
5 before_action :load_task, only: %i[show update destroy]
6 # previous code...In addition to before filters, you can also run filters after an action has
been executed. The after filters are registered via after_action.
These Rails filters help us keep the code DRY.
The above code will ensure we get an error if policy_scope or authorize
methods were not called during execution of the action. Refer the official
documentation of
verify_authorized
and
verify_policy_scoped
for better insight.
The reason we call verify_authorized and verify_policy_scoped with
after_action is because we can only check whether policies have been applied
or not, only after the policies have had an opportunity to act i.e. that the
controller action has run completely. Thus we can only use the after filter.
You can learn more about how Pundit helps in ensuring policies and scopes are used from the official documentation.
We have successfully implemented authorization in our application.
Now let's commit these changes:
1git add -A
2git commit -m "Added pundit task policy"