Backbone.js collection 'freshen' method that respects local changes.

backbone.basecollection.js

  Backbone.BaseCollection = Backbone.Collection.extend({
    // Holds id of locally deleted items we ignore if we 
    // 'freshen' the collection and changes haven't propagated yet.
    _localDeletedIds: [],
    // Take an array of raw objects
    // If the ID matches a model in the collection, set that model
    // If the ID is not found in the collection, add it
    // If a model in the collection is no longer available, remove it
    //
    // Keeps local changes, in case we've added things in the meantime.
    freshen: function(objects) {
      var model;
      // Mark all for removal, unless local only change
      this.each(function(m) {
        if (!m.isNew()) m._remove = true;
      });
      // Apply each object
      _(objects).each(function(attrs) {
        model = this.get(attrs.id);
        if (model) {
          model.set(attrs); // existing model
          delete model._remove
        } else {
          // add new models, accounting for local deletions
          var locallyDeleted = _.find(this._localDeletedIds, 
            function(id){ return id == attrs.id });
          if (!locallyDeleted) this.add(attrs);
        }
      }, this);
      // Now check for any that are still marked for removal
      var toRemove = this.filter(function(m) {
        return m._remove;
      })
      _(toRemove).each(function(m) {
        this.remove(m);
      }, this);
      this.trigger('freshen', this);
    },
    remove: function(models, options) {
      models = _.isArray(models) ? models.slice() : [models];
      for (i = 0, l = models.length; i < l; i++) {
        if (models[i].id) this._localDeletedIds.push(models[i].id);
      }
      return Backbone.Collection.prototype.remove.call(this, models, options); 
    }
  });