custom relations and "monadic" relations

Readme.md

@@ -8,24 +8,48 @@ Backbone.Rel exposes a method `rel` that is a relationship *getter*.
 You can implement your `hasMany` and `belongsTo` like in this example:
 
 ``` javascript
+// models/project.js
+Models.Project.hasMany = function () {
+  return {
+    users: {collection: Collections.users, id: 'project_id'}
+  , tasks: {collection: Collection.tasks, filter: function (task) {
+      return task.rel('project') = this;
+    }}
+  };
+};
+
+// models/user.js
 Models.User.hasMany = function () {
   return {
     tasks: {collection: Collections.tasks, id: 'user_id'}
   };
 };
 
+Models.User.belongsTo = function () {
+  return {
+    project: Collection.projects
+  };
+};
+
+// models/task.js
 Models.Task.belongsTo = function () {
   return {
     user: Collections.users
+  , project: function (task) {
+      return Collection.projects.get(task.rel('user').project);
+    }
   };
 };
 
-var user = new User({id: 1})
+var project = new Project({id: 1})
+  , user = new User({id: 1, project_id: 1})
   , task1 = new Task({id: 1, user_id: 1})
   , task2 = new Task({id: 2, user_id: 1});
 
 assert.equal(user.rel('tasks').length, 2);
+assert.equal(user.rel('project'), project);
 assert.equal(task1.rel('user'), user);
+assert.equal(task1.rel('project'), project);
 ```
 
 ## Dependencies

backbone.rel.js

@@ -21,8 +21,22 @@
    */
   function rel(key, options) {
     var self = this
+      , keys = key.split('.')
       , singularize;
 
+    // kind of monadic accesor
+    if (keys.length > 1) {
+      return _.reduce(keys, function (memo, key) {
+        if (typeof memo === 'undefined') {
+          return self.rel(key);
+        } else if (memo) {
+          return memo.rel(key);
+        } else {
+          return null;
+        }
+      }, undefined);
+    }
+
     options = options || {};
 
     // poor singularize fallback
@@ -37,10 +51,12 @@
 
       var options = self.hasMany()[key];
 
+      function filter(el) {
+        return el.get(options.id) === this.id;
+      }
+
       if (options) {
-        return options.collection.filter(function (el) {
-          return el.get(options.id) === self.id;
-        });
+        return options.collection.filter(_.bind(options.filter || filter, self));
       } else {
         return null;
       }
@@ -51,10 +67,14 @@
         return null;
       }
 
-      var collection = self.belongsTo()[key];
+      var target = self.belongsTo()[key];
 
-      if (collection) {
-        return collection.get(self.get(singularize(key) + '_id'));
+      if (target) {
+        if (_.isFunction(target)) {
+          return target(self) || null;
+        } else {
+          return target.get(self.get(singularize(key) + '_id')) || null;
+        }
       } else {
         return null;
       }

test/test.js

@@ -14,7 +14,9 @@ Models.Task = Backbone.Model.extend({
   belongsTo: function () {
     return {
       user: users
-    , project: projects
+    , project: function (task) {
+        return projects.get(task.rel('user.project'));
+      }
     };
   }
 });
@@ -36,7 +38,11 @@ Models.User = Backbone.Model.extend({
 Models.Project = Backbone.Model.extend({
   hasMany: function () {
     return {
-      tasks: {collection: tasks, id: 'project_id'}
+      tasks: {collection: tasks, filter: function (task) {
+        return task.rel('project')
+          ? task.rel('project').id === this.id
+          : null;
+      }}
     , users: {collection: users, id: 'project_id'}
     };
   }
@@ -72,7 +78,11 @@ describe('Rel', function () {
     }
 
     for (var i = 0; i < 6; i++) {
-      tasks.add({id: i, user_id: i % 2, project_id: i % 3});
+      if (i === 0) {
+        tasks.add({id: i});
+      } else {
+        tasks.add({id: i, user_id: i % 2});
+      }
     }
 
     for (var i = 0; i < 2; i++) {
@@ -83,11 +93,11 @@ describe('Rel', function () {
   it('returns the project for a given user', function () {
     assert.equal(users.get(0).rel('project'), projects.get(0));
     assert.equal(users.get(1).rel('project'), projects.get(1));
-    assert.deepEqual(users.get(2).rel('project'), undefined);
+    assert.deepEqual(users.get(2).rel('project'), null);
   });
 
   it('returns the tasks for a given user', function () {
-    assert.deepEqual(_.pluck(users.get(0).rel('tasks'), 'id'), [0, 2, 4]);
+    assert.deepEqual(_.pluck(users.get(0).rel('tasks'), 'id'), [2, 4]);
     assert.deepEqual(_.pluck(users.get(1).rel('tasks'), 'id'), [1, 3, 5]);
     assert.deepEqual(_.pluck(users.get(2).rel('tasks'), 'id'), []);
   });
@@ -99,18 +109,21 @@ describe('Rel', function () {
   });
 
   it('returns the user for a given task', function () {
-    assert.deepEqual(tasks.get(0).rel('user'), users.get(0));
+    assert.deepEqual(tasks.get(0).rel('user'), null);
     assert.deepEqual(tasks.get(1).rel('user'), users.get(1));
     assert.deepEqual(tasks.get(2).rel('user'), users.get(0));
     assert.deepEqual(tasks.get(3).rel('user'), users.get(1));
+    assert.deepEqual(tasks.get(4).rel('user'), users.get(0));
     // ...
   });
 
   it('returns the project for a given task', function () {
-    assert.deepEqual(tasks.get(0).rel('project'), projects.get(0));
+    assert.throws(function () {
+      assert.deepEqual(tasks.get(0).rel('project'), projects.get(0));
+    });
     assert.deepEqual(tasks.get(1).rel('project'), projects.get(1));
-    assert.deepEqual(tasks.get(2).rel('project'), undefined);
-    assert.deepEqual(tasks.get(3).rel('project'), projects.get(0));
+    assert.deepEqual(tasks.get(2).rel('project'), projects.get(0));
+    assert.deepEqual(tasks.get(3).rel('project'), projects.get(1));
     // ...
   });
 
@@ -120,8 +133,8 @@ describe('Rel', function () {
   });
 
   it('returns the tasks for a given project', function () {
-    assert.deepEqual(_.pluck(projects.get(0).rel('tasks'), 'id'), [0, 3]);
-    assert.deepEqual(_.pluck(projects.get(1).rel('tasks'), 'id'), [1, 4]);
+    assert.deepEqual(_.pluck(projects.get(0).rel('tasks'), 'id'), [2, 4]);
+    assert.deepEqual(_.pluck(projects.get(1).rel('tasks'), 'id'), [1, 3, 5]);
   });
 
   it('returns the users for a given project', function () {