code: function(model) {
          var modelObj = model.changedAttributes();
          _.each(model.changed, function(value, key) {
            if (typeof value === 'undefined' || value === null) {
              if (key == 'id') {
                delete modelObj[key];
              } else {
                modelObj[key] = null;
              }
            }
          });
  
          return modelObj;
        }
docs: Find the deleted keys and set their values to null
  so Firebase properly deletes them.