Return id assumption for simple polymorphic Collection#model factories.

Also, add tests to make sure it doesn't break in the future.

backbone.js

@@ -703,6 +703,7 @@
       var toAdd = [], toRemove = [], modelMap = {};
       var add = options.add, merge = options.merge, remove = options.remove;
       var order = !sortable && add && remove ? [] : false;
+      var targetProto = this.model.prototype;
 
       // Turn bare objects into model references, and prevent invalid models
       // from being added.
@@ -710,8 +711,10 @@
         attrs = models[i] || {};
         if (attrs instanceof Model) {
           id = model = attrs;
+        } else if (targetProto.generateId) {
+          id = targetProto.generateId(attrs);
         } else {
-          id = this.model.prototype.generateId(attrs);
+          id = attrs[targetProto.idAttribute || Model.prototype.idAttribute];
         }
 
         // If a duplicate is found, prevent it from being added and

test/collection.js

@@ -1362,4 +1362,53 @@
     equal(collection.get(1), collection.first());
   });
 
+  test('Polymorphic models work with "simple" constructors', function () {
+    var A = Backbone.Model.extend();
+    var B = Backbone.Model.extend();
+    var C = Backbone.Collection.extend({
+      model: function (attrs) {
+        return attrs.type === 'a' ? new A(attrs) : new B(attrs);
+      }
+    });
+    var collection = new C([{id: 1, type: 'a'}, {id: 2, type: 'b'}]);
+    equal(collection.length, 2);
+    ok(collection.at(0) instanceof A);
+    equal(collection.at(0).id, 1);
+    ok(collection.at(1) instanceof B);
+    equal(collection.at(1).id, 2);
+  });
+
+  test('Polymorphic models work with "advanced" constructors', function () {
+    var A = Backbone.Model.extend();
+    var B = Backbone.Model.extend();
+    var C = Backbone.Collection.extend({
+      model: Backbone.Model.extend({
+        constructor: function (attrs) {
+          return attrs.type === 'a' ? new A(attrs) : new B(attrs);
+        },
+
+        idAttribute: '_id'
+      })
+    });
+    var collection = new C([{_id: 1, type: 'a'}, {_id: 2, type: 'b'}]);
+    equal(collection.length, 2);
+    ok(collection.at(0) instanceof A);
+    ok(collection.at(1) instanceof B);
+
+    C = Backbone.Collection.extend({
+      model: Backbone.Model.extend({
+        constructor: function (attrs) {
+          return attrs.type === 'a' ? new A(attrs) : new B(attrs);
+        },
+
+        generateId: function (attrs) {
+          return attrs.type + '-' + attrs.id;
+        }
+      })
+    });
+    collection = new C([{id: 1, type: 'a'}, {id: 1, type: 'b'}]);
+    equal(collection.length, 2);
+    ok(collection.at(0) instanceof A);
+    ok(collection.at(1) instanceof B);
+  });
 })();