Ionic and Angularjs - form binding appears to be only one way.?

I've a list of items. They are objects held in itemArray. When you touch one you it fires ng-click="openEditor(item)". Like this:

<ion-list>
  <ion-item ng-repeat="item in ItemArray" class="item item-avatar" ng-click="openEditor(item)">
    <img ng-src="{{iconMap[item.type]}}">
    <h2>{{item.dateString}}</h2><span>{{item.startTimeString}}</span>
    <p>ASA {{item.priority}}: {{item.itemType}}</p>
  </ion-item>
</ion-list>

Then this happens:

$scope.openEditor =function(item){ 
   $scope.currentItem = item;
   $scope.modal.show();
}

and in the modal I have

<select ng-model="currentItem.priority">
          <option value="1">1</option>
          <option value="2">2</option>
          <option value="3">3</option>
          <option value="4">4</option>
          <option value="5">5</option>
        </select>

It shows the correct priority in the dropdown (I've tested it with a lot of the items).

The problem is - I cannot change it! If I try I can't alter the option in the dropdown.

I get this error:

TypeError: Cannot assign to read only property 'priority' of #<Object>

Why is this happening? I want to change the priority and then save it back into the itemArray and SQL DB.

Sounds like the object you're trying to change is read-only. You need to investigate why this is, but you could make a copy of the object with

$scope.openEditor =function(item){ 
   $scope.currentItem = angular.copy(item);
   $scope.modal.show();
}