Django REST - Access Policy
===========================

|Package version| |Python versions|

This project brings a declaritive, organized approach to managing access
control in Django REST Framework projects. Each ViewSet or
function-based view can be assigned an explicit policy for the exposed
resource(s). No more digging through views or seralizers to understand
access logic -- it's all in one place in a format that less technical
stakeholders can understand. If you're familiar with other declaritive
access models, such as AWS' IAM, the syntax will be familiar.

In short, you can start expressing your access rules like this:

.. code:: python

    class ArticleAccessPolicy(AccessPolicy):
        statements = [
            {
                "action": ["list", "retrieve"],
                "principal": "*",
                "effect": "allow"
            },
            {
                "action": ["publish", "unpublish"],
                "principal": ["group:editor"],
                "effect": "allow"            
            },
        ]

This project has complete test coverage and the base ``AccessPolicy``
class is only ~150 lines of code: there's no magic here.

Table of Contents:
==================

-  `Installation <#installation>`__
-  `Example #1: Policy for ViewSet <#example-1-policy-for-viewset>`__
-  `Example #2: Policy for Function-Based
   View <#example-2-policy-for-function-based-view>`__
-  `Documentation <#documentation>`__
-  `Statement Elements <#statement-elements>`__
-  `Policy Evaluation Logic <#policy-evaluation-logic>`__
-  `Object-Level Permissions/Conditions <#object-level-perm>`__
-  `Multitenancy Data/Restricting
   QuerySets <#multitenancy-data--restricting-querysets>`__
-  `Attaching to ViewSets and Function-Based
   Views <#attaching-to-viewsets-and-function-based-views>`__
-  `Loading Statements from External
   Source <#loading-statements-from-external-source>`__
-  `Customizing User Group/Role
   Values <#customizing-user-grouprole-values>`__
-  `Customizing Principal Prefixes <#customizing-principal-prefixes>`__
-  `Changelog <#changelog>`__
-  `Testing <#testing>`__
-  `License <#license>`__

Setup
=====

::

    pip install drf-access-policy

To define a policy, import ``AccessPolicy`` and subclass it:

.. code:: python

    from rest_access_policy import AccessPolicy


    class ShoppingCartAccessPolicy(AccessPolicy):
        statements = [] # Now read on...

Example #1: Policy for ViewSet
==============================

In a nutshell, a policy is comprised of "statements" that declare what
"actions" a "principal" can or cannot perform on the resource, with
optional custom checks that can examine any detail of the current
request.

Here are two more key points to remember going forward: \* all access is
implicitly denied by default \* any statement with the "deny" effect
overrides any and all "allow" statement

Now let's look at the policy below an articles endpoint, provided
through a view set.

.. code:: python

    class ArticleAccessPolicy(AccessPolicy):
        statements = [
            {
                "action": ["list", "retrieve"],
                "principal": "*",
                "effect": "allow"
            },
            {
                "action": ["publish", "unpublish"],
                "principal": ["group:editor"],
                "effect": "allow"            
            },
            {
                "action": ["delete"],
                "principal": ["*"],
                "effect": "allow",
                "condition": "is_author"         
            },
            {
                "action": ["*"],
                "principal": ["*"],
                "effect": "deny",
                "condition": "is_happy_hour"
            }
        ]

        def is_author(self, request, view, action) -> bool:
            article = view.get_object()
            return request.user == article.author 

        def is_happy_hour(self, request, view, action) -> bool:
            now = datetime.datetime.now()
            return now.hour >= 17 and now.hour <= 18:

        @classmethod
        def scope_queryset(cls, request, queryset):
            if request.user.groups.filter(name='editor').exists():
                return queryset

            return queryset.filter(status='published')


    class ArticleViewSet(ModelViewSet):
        # Just stick the policy here, as you would do with
        # regular DRF "permissions"
        permission_classes = (ArticleAccessPolicy, )

        # Helper property here to make get_queryset logic
        # more explicit
        @property
        def access_policy(self):
            return self.permission_classes[0]

        # Ensure that current user can only see the models 
        # they are allowed to see
        def get_queryset(self):
            return self.access_policy.scope_queryset(
                self.request, Articles.objects.all()
            )
        
        @action(method="POST")
        def publish(self, request, *args, **kwargs):
            pass

        @action(method="POST")
        def unpublish(self, request, *args, **kwargs):
            pass

        # the rest of you view set definition...

The actions correspond to the names of methods on the ViewSet.

In the example above, the following rules are put in place: - anyone is
allowed to list and retrieve articles - users in the editor group are
allowed to publish and unpublish articles - in order to delete an
article, the user must be the author of the article. Notice how the
condition method ``is_author`` calls ``get_object()`` on the view to get
the current article. - if the condition ``is_happy_hour``, evaluates to
``True``, then no one is allowed to do anything.

Additionally, we have some logic in the ``scope_queryset`` method for
filtering which models are visible to the current user. Here, we want
users to only see published articles, unless they are an editor, in
which case they case see articles with any status. You have to remember
to call this method from the view, so I'd suggest reviewing this as part
of a security audit checklist.

Example #2: Policy for Function-Based View
==========================================

You can also you policies with function-based views. The action to
reference in your policy statements is the name of the function. You can
also bundle multiple functions into the same policy as the example below
shows.

.. code:: python

    class AuditLogsAccessPolicy(AccessPolicy):
        statements = [
            {
                "action": ["search_logs"],
                "principal": "group:it_staff",
                "effect": "allow"
            },
            {
                "action": ["download_logs"],
                "principal": ["group:it_admin"],
                "effect": "allow"            
            }
        ]


    @api_view(["GET"])
    @permission_classes((AuditLogsAccessPolicy,))
    def search_logs(request):
        ## you logic here...
        pass


    @api_view(["GET"])
    @permission_classes((AuditLogsAccessPolicy,))
    def download_logs(request):
        ## you logic here...
        pass

Documentation
=============

Statement Elements
------------------

+-----------------+-----------------------------------------+------------------+
| Name            | Description                             | Examples         |
+=================+=========================================+==================+
| principal       | Identifies the user of the current      | ``["group:admins |
|                 | request by the name of a group they     | "]``             |
|                 | belong to or their user id. Can be      | ``["id:5352"]``  |
|                 | formatted as a string or list of        |                  |
|                 | strings. Use an asterisk ("\*") if you  |                  |
|                 | want the statement to apply to all      |                  |
|                 | principals.                             |                  |
+-----------------+-----------------------------------------+------------------+
| action          | The action or actions that the          | ``["list", "dele |
|                 | statement applies to. Can be formatted  | te", "create]``  |
|                 | as a string or list of strings. The     | ``["*"]``        |
|                 | value should match the name of a view   |                  |
|                 | set method or the name of the view      |                  |
|                 | function. Use an asterisk ("\*") if you |                  |
|                 | want the statement to apply to all      |                  |
|                 | actions.                                |                  |
+-----------------+-----------------------------------------+------------------+
| effect          | Whether the statement, if it is in      | ``"allow"``      |
|                 | effect, should allow or deny access.    | ``"deny"``       |
|                 | All access is denied by default, so use |                  |
|                 | ``deny`` when you'd like to override an |                  |
|                 | ``allow`` statement that will also be   |                  |
|                 | in effect. Should be a string equal to  |                  |
|                 | either ``deny`` or ``allow``.           |                  |
+-----------------+-----------------------------------------+------------------+
| condition       | The name of a method on the policy that | ``"is_manager_fo |
|                 | returns a boolean. The method signature | r_account"``     |
|                 | is                                      | ``"is_author_of_ |
|                 | ``condition(request, view, action: str) | post"``          |
|                 | ``.                                     | ``["balance_is_p |
|                 | If true, the policy will be in effect.  | ositive", "accou |
|                 | Useful for enforcing object-level       | nt_is_not_frozen |
|                 | permissions. If list of strings, all    | "]``             |
|                 | conditions must evaluate to ``True``.   |                  |
+-----------------+-----------------------------------------+------------------+

Policy Evaluation Logic
-----------------------

To determine whether access to a request is granted, all applicable
statements are first filtered. A statement is applicable to the current
request if all of the following are true (1) the request user matches
one of the statement's principals, (2) the name of the method/function
matches one of its actions, and (3) any custom conditions evaluate to
true.

The request is allowed if any of the statements have an effect of
"allow", and none have an effect of "deny". By default, all requests are
denied.

Object-Level Permissions/Conditions  
------------------------------------

You may be wondering, but what object-level permissions? Not to worry -
you can easily check object-level access in a custom condition that's
evaluated to determine whether the statement takes effect. This
condition is passed the ``view`` instance, so you can easily get the
model instance with a call to ``view.get_object()``. You can even
reference multiple conditions, to keep your access methods focused and
easy to test.

.. code:: python

    class AccountAccessPolicy(AccessPolicy):
        statements = [
            ## ... other statements ...
            {
                "action": ["withdraw"],
                "principal": ["*"],
                "effect": "allow",
                "condition": ["balance_is_positive", "is_account_owner"]     
            }
            ## ... other statements ...
        ]

        def balance_is_positive(self, request, view, action) -> bool:
            account = view.get_object()
            return account.balance > 0

        def is_account_owner(self, request, view, action) -> bool:
            account = view.get_object()
            return account.owner == request.user

Multitenancy Data / Restricting QuerySets
-----------------------------------------

You can define a class method on your policy class that takes a QuerySet
and the current request and returns a securely scoped QuerySet
representing only the database rows that the current user should have
access to. This is helpful for multitenant situations or more generally
when users should not have full visibility to model instances. Of course
you could do this elsewhere in your code, but putting this method on the
policy class keeps all access logic in a single place.

.. code:: python

        class PhotoAlbumAccessPolicy(AccessPolicy):
            # ... statements, etc ...

            # Users can only access albums they have created
            @classmethod
            def scope_queryset(cls, request, qs):
                return qs.filter(creator=request.user)


        class TodoListAccessPolicy(AccessPolicy):
            # ... statements, etc ...

            # Users can only access todo lists owned by their organization
            @classmethod
            def scope_queryset(cls, request, qs):
                user_orgs = request.user.organizations.all()
                return qs.filter(org__id__in=user_orgs)

Attaching to ViewSets and Function-Based Views
----------------------------------------------

You attach access policies the same way you do with regular DRF
permissions.

For ViewSets, add it to ``permissions`` property:

.. code:: python

    class ArticleViewSet(ModelViewSet):
        permission_classes = (ArticleAccessPolicy, )

For function-based views, add it to ``permissions_classes`` decorator:

.. code:: python

    @api_view(["GET"])
    @permission_classes((ArticleAccessPolicy,))
    def create_article(request):
        ## you logic here...
        pass

Loading Statements from External Source
---------------------------------------

If you don't want your policy statements hardcoded into the classes, you
can load them from an external data source: a great step to take because
you can then change access rules without redeploying code.

Just define a method on your policy class called
``get_policy_statements``, which has the following signature:
``get_policy_statements(self, request, view) -> List[dict]``

Example:

.. code:: python

    class UserAccessPolicy(AccessPolicy):
        id = 'user-policy'

        def get_policy_statements(self, request, view) -> List[dict]:
            statements = data_api.load_json(self.id)
            return json.loads(statements)

You probably want to only define this method once on your own custom
subclass of ``AccessPolicy``, from which all your other access policies
inherit.

Customizing User Group/Role Values
----------------------------------

If you aren't using Django's built-in auth app, you may need to define a
custom way to retrieve the role/group names to which the user belongs.
Just define a method called ``get_user_group_values`` on your policy
class. It is passed a single argument: the user of the current request.
In the example below, the user model has a to-many relationship with a
"roles", which have their "name" value in a field called "title".

.. code:: python

    class UserAccessPolicy(AccessPolicy):
        # ... other properties and methods ...

        def get_user_group_values(self, user) -> List[str]:
            return list(user.roles.values_list("title", flat=True))

Customizing Principal Prefixes
------------------------------

By default, the prefixes to identify the type of principle (user or
group) are "id:" and "group:", respectively. You can customize this by
setting these properties on your policy class:

.. code:: python

    class FriendRequestPolicy(permissions.BasePermission):
        group_prefix = "role:"
        id_prefix = "staff_id:"

        # .. the rest of you policy definition ..

Changelog 
==========

0.1.0 (May 2019)
----------------

-  Initial release

Testing
=======

Tests are found in a simplified Django project in the ``/tests`` folder.
Install the project requirements and do ``./manage.py test`` to run
them.

License
=======

See `License <LICENSE.md>`__.

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