Activity based permissions with Backbone, API design ideas?

Question

I'm building a fairly large cms-type application with Backbone and Knockout and Knockback (ko + bb bridge library), and I'm trying to figure out a good way to abstract permissions. Also sorry in advance for the novel.

First of all, this is a pretty non-standard architecture, and the second question you might ask - why don't you use something more comprehensive like Ember or Angular? Point taken. It is what it is at this point. :)

So here's my quandary. I want an elegant api at both the controller and viewmodel level for permissions.

I have an object available to me that looks like this:

{
   'api/pages': {
     create: true, read: true, update: true, destroy: true
   },
   'api/links': {
     create: false, read: true, update: false, destroy: false
   }
   ...
}

So in my router/controllers, I'm newing up my collections/models/viewmodels, and then calling a customized render method on an already existing view. The view takes care of things like releasing the viewmodels.

initialize: function() {
  this.pages = new PagesCollection();
  this.links = new LinksCollection();
},

list: function() {
  var vm = new PageListViewmodel(this.pages, this.links);
  // adminPage method is available through inheritance
  this.adminPage('path/to/template', vm); // delegates to kb.renderTemplate under the hood.
}

So the problem with this, are these collections are totally unstructured, ie. the router doesn't know anything about them.

But what I need is for it to redirect to an unauthorized page if you're not allowed to view a particular resource.

So with the example above, I've thought about coding in before/after filters? But where would you specify what each router method is trying to access?

list: function() {
  this.authorize([this.pages, this.links], ['read'], function(pages, links) {
    // return view.
  });
}

The previous code is really cludgy..

For the viewmodels, which are more straightforward I had the idea of doing something like this - ala Ruby's CanCan:

this.currentUser.can('read', collection) // true or false
// can() would just look at the endpoint and compare to my perms object.

Answer 1

You could extend your router to wrap your routes callbacks to perform a validity check before allowing the action.

var Router = Backbone.Router.extend({
    routes: {
        "app/*perm": "go"
    },

    route: function(route, name, callback) {
        if (!callback) callback = this[name];

        var f = function() {
            var perms = this.authorized(Backbone.history.getFragment());
            if (perms === true) {
                callback.apply(this, arguments);
            } else {
                this.trigger('denied', perms);
            }
        };
        return Backbone.Router.prototype.route.call(this, route, name, f);
    },

    authorized: function(path) {
        // check if the path is authorized
    },

    go: function(perm) {
       // perform action
    }
});

If the path is authorized, the route performs as usual, a denied event is triggered if not.

The authorized method could be based on a list of paths mapped to your permissions objects, something like this

var permissions = {
   'api/pages': {
     create: true, read: true, update: true, destroy: true
   },
   'api/links': {
     create: false, read: true, update: false, destroy: false
   }
}
var Router = Backbone.Router.extend({
    routes: {
        "app/*perm": "go"
    },

    // protected paths, with the corresponding entry in the permissions object
    permissionsMap: {
        "app/pages": 'api/pages',
        "app/links": 'api/links',
    },

    route: function(route, name, callback) {
        // see above
    },

    // returns true if the path is allowed
    // returns an object with the path and the permission key used if not
    authorized: function(path) {
        var paths, match, permkey, perms;

        // find an entry for the current path
        paths = _.keys(this.permissionsMap);
        match = _.find(paths, function(p) {
            return path.indexOf(p)===0;            
        });
        if (!match) return true;

        //check if the read permission is allowed
        permkey = this.permissionsMap[match];
        if (!permissions[permkey]) return true;
        if (permissions[permkey].read) return true;

        return {
            path: path,
            permission: permkey
        };
    },

    go: function(perm) {}
});

And a demo http://jsfiddle.net/t2vMA/1/

Answer 2

Nikoshr's answer gave me something to run with. I didn't think to actually override route itself. But here's my solution. I should have mentioned it in the question - but sometimes a router action requires more than one collection.

The code on this is really rough, and needs tests - but it works! Fiddle here.

Here are the relevant portions - These two methods take care of the authorization.

authorize: function(namedRoute) {
  if (this.permissions && this.collections) {
    var perms = this.permissions[namedRoute];
    if (!perms) {
      perms = {};
      // if nothing is specified for a particular  route, we
      // assume read access required for all registered controllers.
      _.each(_.keys(this.collections), function(key) {
        return perms[key] = [];
      });
    }

    var authorized = _.chain(perms)
      .map(function(reqPerms, collKey) {
        var collection = this.collections[collKey],
            permKey = _.result(collection, 'url');

        // We implicitly check for 'read'
        if (!_.contains('read')) {
          reqPerms.push('read');
        }

        return _.every(reqPerms, function(ability) {
          return userPermissions[permKey][ability];
        });
      }, this)
      .every(function(auth){ return auth; })
      .value();
    return authorized;
  }
  return true;
},
route: function(route, name, callback) {
  if (!callback) { callback = this[name]; }
  var action = function() {
    // allow anonymous routes through auth check.
    if (!name || this.authorize(name)) {
      callback.apply(this, arguments);
    } else {
      this.trigger('denied');
    }
  }
  Backbone.Router.prototype.route.call(this, route, name, action);
  return this;
}

And each controller/router inherits from the perm router, where the permissions for each action are mapped like so:

// Setup
routes: {
  'list'     : 'list',
  'list/:id' : 'detail',
  'create'   : 'create'
},

// Collection are registered so we can
// keep track of what actions use them
collections: {
  pages: new PagesCollection([{id:1, title: 'stuff'}]),
  links: new LinksCollection([{id:1, link: 'things'}])
},

// If a router method is not defined,
// 'read' access is assumed to be
// required for all registered collections.
permissions: {
  detail: {
    pages: ['update'],
    links: ['update']
  },
  create: {
    pages: ['create'],
    links: ['create', 'update']
  }
},