code: function(sPropertyName, sSortOrder) {
  			var oProperty = this._oEntityType.findPropertyByName(sPropertyName);
  			if (oProperty == null) {
  				throw "Cannot add sort condition for unknown property name " + sPropertyName; // TODO
  			}
  			var oExistingSorterEntry = this._containsSorter(sPropertyName);
  			if (oExistingSorterEntry != null) {
  				oExistingSorterEntry.sorter.order = sSortOrder;
  				return this;
  			}
  			var aSortablePropertyNames = this._oEntityType.getSortablePropertyNames();
  			if (((aSortablePropertyNames ? Array.prototype.indexOf.call(aSortablePropertyNames, sPropertyName) : -1)) === -1) {
  				throw "Cannot add sort condition for not sortable property name " + sPropertyName; // TODO
  			}
  
  			this._aSortCondition.push({
  				property : oProperty,
  				order : sSortOrder
  			});
  			return this;
  		}
docs: Add a condition to the order by expression. It replaces any previously specified
  sort order for the property.
  
  @param {string}
  sPropertyName The name of the property bound in the condition
  @param {sap.ui.model.analytics.odata4analytics.SortOrder}
  sSortOrder sorting order used for the condition
  @throws Exception
  if the property is unknown, not sortable or already added as
  sorter
  @returns {sap.ui.model.analytics.odata4analytics.SortExpression} This object for method
  chaining
  @public
  @function
  @name sap.ui.model.analytics.odata4analytics.SortExpression#addSorter