code: function(oContext) {
  			// use the id of the ManagedObject instance as the identifier
  			// for the extended change detection
  			var oObject = oContext.getObject();
  			if (oObject instanceof  ManagedObject) {
  				return oObject.getId();
  			}
  
  			return JSONListBinding.prototype.getEntryKey.apply(this, arguments);
  		}
docs: Use the id of the ManagedObject instance as the unique key to identify
  the entry in the extended change detection. The default implementation
  in the parent class which uses JSON.stringify to serialize the instance
  doesn't fit here because none of the ManagedObject instance can be
  Serialized.
  
  @param {sap.ui.model.Context} oContext the binding context object
  @return {string} The identifier used for diff comparison
  @see sap.ui.model.ListBinding.prototype.getEntryData