An authorization library in Elixir for Plug applications that restricts what resources the current user is allowed to access, and automatically loads resources for the current request.
Inspired by CanCan for Ruby on Rails.
For the latest master:
defp deps do
{:canary, github: "cpjk/canary"}
end
For the latest release:
defp deps do
{:canary, "~> 1.0.0"}
end
Then run mix deps.get
to fetch the dependencies.
Canary provides three functions to be used as plugs to load and authorize resources:
load_resource/2
, authorize_resource/2
, and load_and_authorize_resource/2
.
load_resource/2
and authorize_resource/2
can be used by themselves, while load_and_authorize_resource/2
combines them both.
In order to use Canary, you will need, at minimum:
-
A Canada.Can protocol implementation (a good place would be
lib/abilities.ex
) -
An Ecto record struct containing the user to authorize in
conn.assigns.current_user
(the key can be customized - see https://github.com/cpjk/canary#overriding-the-default-user). -
Your Ecto repo specified in your
config/config.exs
:config :canary, repo: YourApp.Repo
Then, just import Canary.Plugs
in order to use the plugs. In a Phoenix app the best place would probably be inside controller/0
in your web/web.ex
, in order to make the functions available in all of your controllers.
Loads the resource having the id given in conn.params["id"]
from the database using the given Ecto repo and model, and assigns the resource to conn.assigns.<resource_name>
, where resource_name
is inferred from the model name.
For example,
plug :load_resource, model: Project.Post
Will load the Project.Post
having the id given in conn.params["id"]
through YourApp.Repo
, into
conn.assigns.post
Checks whether or not the current_user
for the request can perform the given action on the given resource and assigns the result (true/false) to conn.assigns.authorized
. It is up to you to decide what to do with the result.
For Phoenix applications, Canary determines the action automatically.
For non-Phoenix applications, or to override the action provided by Phoenix, simply ensure that conn.assigns.canary_action
contains an atom specifying the action.
In order to authorize resources, you must specify permissions by implementing the Canada.Can protocol for your User
model (Canada is included as a light weight dependency).
Authorizes the resource and then loads it if authorization succeeds. Again, the resource is loaded into conn.assigns.<resource_name>
.
In the following example, the Post
with the same user_id
as the current_user
is only loaded if authorization succeeds.
Let's say you have a Phoenix application with a Post
model, and you want to authorize the current_user
for accessing Post
resources.
Let's suppose that you have a file named lib/abilities.ex
that contains your Canada authorization rules like so:
defimpl Canada.Can, for: User do
def can?(%User{ id: user_id }, action, %Post{ user_id: user_id })
when action in [:show], do: true
def can?(%User{ id: user_id }, _, _), do: false
end
and in your web/router.ex:
you have:
get "/posts/:id", PostController, :show
delete "/posts/:id", PostController, :delete
To automatically load and authorize on the Post
having the id
given in the params, you would add the following plug to your PostController
:
plug :load_and_authorize_resource, model: Post
In this case, on GET /posts/12
authorization succeeds, and the Post
specified by conn.params["id]
will be loaded into conn.assigns.post
.
However, on DELETE /posts/12
, authorization fails and the Post
resource is not loaded.
To exclude an action from any of the plugs, pass the :except
key, with a single action or list of actions.
For example,
Single action form:
plug :load_and_authorize_resource, model: Post, except: :show
List form:
plug :load_and_authorize_resource, model: Post, except: [:show, :create]
To specify that a plug should be run only for a specific list of actions, pass the :only
key, with a single action or list of actions.
For example,
Single action form:
plug :load_and_authorize_resource, model: Post, only: :show
List form:
plug :load_and_authorize_resource, model: Post, only: [:show, :create]
Note: Passing both :only
and :except
to a plug is invalid. Canary will simply pass the Conn
along unchanged.
Globally, the default key for finding the user to authorize can be set in your configuration as follows:
config :canary, current_user: :some_current_user
In this case, canary will look for the current user record in conn.assigns.some_current_user
.
The current user key can also be overridden for individual plugs as follows:
plug :load_and_authorize_resource, model: Post, current_user: :current_admin
To specify the name under which the loaded resource is stored, pass the :as
flag in the plug declaration.
For example,
plug :load_and_authorize_resource, model: Post, as: :new_post
will load the post into conn.assigns.new_post
Associations can be preloaded with Repo.preload
by passing the :preload
option with the name of the association:
plug :load_and_authorize_resource, model: Post, preload: :comments
For the :index
, :new
, and :create
actions, the resource passed to the Canada.Can
implementation
should be the module name of the model rather than a struct.
For example, when authorizing access to the Post
resource,
you should use
def can?(%User{}, :index, Post), do: true
instead of
def can?(%User{}, :index, %Post{}), do: true
You may wish to define permissions for when there is no logged in current user (when conn.assigns.current_user
is nil
).
In this case, you should implement Canada.Can
for nil
like so:
defimpl Canada.Can, for: Atom do
# When the user is not logged in, all they can do is read Posts
def can?(nil, :show, %Post{}), do: true
def can?(nil, _, _), do: false
end
Sometimes you need to load and authorize a parent resource when you have a relationship between two resources and you are
creating a new one or listing all the children of that parent. By specifying the :persisted
option with true
you can load and/or authorize a nested resource. Specifying this option overrides the default loading behavior of the
:index
, :new
, and :create
actions by loading an individual resource. It also overrides the default
authorization behavior of the :index
, :new
, and create
actions by loading a struct instead of a module
name for the call to Canada.can?
.
For example, when loading and authorizing a Post
resource which can have one or more Comment
resources, use
plug :load_and_authorize_resource, model: Post, id_name: "post_id", persisted: true, only: [:create]
to load and authorize the parent Post
resource using the post_id
in /posts/:post_id/comments before you
create the Comment
resource using its parent.
You can tell Canary to search for a resource using a field other than the default :id
by using the :id_field
option. Note that the specified field must be able to uniquely identify any resource in the specified table.
For example, if you want to access your posts using a string field called slug
, you can use
plug :load_and_authorize_resource, model: Post, id_name: "slug", id_field: "slug"
to load and authorize the resource Post
with the slug specified by conn.params["slug"]
value.
If you are using Phoenix, your web/router.ex
should contain something like:
resources "/posts", PostController, param: "slug"
Then your URLs will look like:
/posts/my-new-post
instead of
/posts/1
By default, when an action is unauthorized, Canary simply sets conn.assigns.authorized
to false
.
However, you can configure a handler function to be called when authorization fails. Canary will pass the Plug.Conn
to the given function. The handler should accept a Plug.Conn
as its only argument, and should return a Plug.Conn
.
For example, to have Canary call Helpers.handle_unauthorized/1
:
config :canary, unauthorized_handler: {Helpers, :handle_unauthorized}
By default, when a resource is not found, Canary simply sets the resource in conn.assigns
to nil
. Like unauthorized action handling , you can configure a function to which Canary will pass the conn
when a resource is not found:
config :canary, not_found_handler: {Helpers, :handle_not_found}
You can also specify handlers on an an individual basis (which will override the corresponding configured handler, if any) by specifying the corresponding opt
in the plug call:
plug :load_and_authorize_resource Post,
unauthorized_handler: {Helpers, :handle_unauthorized},
not_found_handler: {Helpers, :handle_not_found}
Tip: If you would like the request handling to stop after the handler function exits, e.g. when redirecting, be sure to call Plug.Conn.halt/1
within your handler like so:
def handle_unauthorized(conn) do
conn
|> put_flash(:error, "You can't access that page!")
|> redirect(to: "/")
|> halt
end
Note: If both an :unauthorized_handler
and a :not_found_handler
are specified for load_and_authorize_resource
, and the request meets the criteria for both, the :unauthorized_handler
will be called first.
MIT License. Copyright 2016 Chris Kelly.