Improve multiple collection support

Preserve original model collection when adding/removing from another collection
Destroying a model removes it from all collections

backbone.js

@@ -287,7 +287,7 @@
       var model = this;
       var success = options.success;
       options.success = function(resp) {
-        if (model.collection) model.collection.remove(model);
+        model.trigger('destroy', model, model.collection, options);
         if (success) success(model, resp);
       };
       options.error = wrapError(options.error, model, options);
@@ -555,7 +555,9 @@
       if (already) throw new Error(["Can't add the same model to a set twice", already.id]);
       this._byId[model.id] = model;
       this._byCid[model.cid] = model;
-      model.collection = this;
+      if (!model.collection) {
+        model.collection = this;
+      }
       var index = this.comparator ? this.sortedIndex(model, this.comparator) : this.length;
       this.models.splice(index, 0, model);
       model.bind('all', this._onModelEvent);
@@ -581,16 +583,21 @@
 
     // Internal method to remove a model's ties to a collection.
     _removeReference : function(model) {
-      delete model.collection;
+      if (this == model.collection) {
+        delete model.collection;
+      }
       model.unbind('all', this._onModelEvent);
     },
 
     // Internal method called every time a model in the set fires an event.
     // Sets need to update their indexes when models change ids. All other
     // events simply proxy through. "add" and "remove" events that originate
     // in other collections are ignored.
-    _onModelEvent : function(ev, model, collection) {
+    _onModelEvent : function(ev, model, collection, options) {
       if ((ev == 'add' || ev == 'remove') && collection != this) return;
+      if (ev == 'destroy') {
+        this._remove(model, options);
+      }
       if (ev === 'change:id') {
         delete this._byId[model.previous('id')];
         this._byId[model.id] = model;

test/collection.js

@@ -73,6 +73,34 @@ $(document).ready(function() {
     ok(opts.amazing);
   });
 
+  test("Collection: add model to multiple collections", function() {
+    var counter = 0;
+    var e = new Backbone.Model({id: 10, label : 'e'});
+    e.bind('add', function(model, collection) {
+      counter++;
+      equals(e, model);
+      if (counter > 1) {
+        equals(collection, colF);
+      } else {
+        equals(collection, colE);
+      }
+    });
+    var colE = new Backbone.Collection([]);
+    colE.bind('add', function(model, collection) {
+      equals(e, model);
+      equals(colE, collection);
+    });
+    var colF = new Backbone.Collection([]);
+    colF.bind('add', function(model, collection) {
+      equals(e, model);
+      equals(colF, collection);
+    });
+    colE.add(e);
+    equals(e.collection, colE);
+    colF.add(e);
+    equals(e.collection, colE);
+  });
+
   test("Collection: remove", function() {
     var removed = otherRemoved = null;
     col.bind('remove', function(model){ removed = model.get('label'); });
@@ -121,6 +149,51 @@ $(document).ready(function() {
     equals(passed, true);
   });
 
+  test("Collection: remove same model in multiple collection", function() {
+    var counter = 0;
+    var e = new Backbone.Model({id: 5, title: 'Othello'});
+    e.bind('remove', function(model, collection) {
+      counter++;
+      equals(e, model);
+      if (counter > 1) {
+        equals(collection, colE);
+      } else {
+        equals(collection, colF);
+      }
+    });
+    var colE = new Backbone.Collection([e]);
+    colE.bind('remove', function(model, collection) {
+      equals(e, model);
+      equals(colE, collection);
+    });
+    var colF = new Backbone.Collection([e]);
+    colF.bind('remove', function(model, collection) {
+      equals(e, model);
+      equals(colF, collection);
+    });
+    equals(colE, e.collection);
+    colF.remove(e);
+    ok(colF.length == 0);
+    ok(colE.length == 1);
+    equals(counter, 1);
+    equals(colE, e.collection);
+    colE.remove(e);
+    equals(null, e.collection);
+    ok(colE.length == 0);
+    equals(counter, 2);
+  });
+
+  test("Colllection: model destroy removes from all collections", function() {
+    var e = new Backbone.Model({id: 5, title: 'Othello'});
+    e.sync = function(method, model, options) { options.success({}); };
+    var colE = new Backbone.Collection([e]);
+    var colF = new Backbone.Collection([e]);
+    e.destroy();
+    ok(colE.length == 0);
+    ok(colF.length == 0);
+    equals(null, e.collection);
+  });
+
   test("Collection: fetch", function() {
     col.fetch();
     equals(lastRequest[0], 'read');