_appendDataFromBackend() {
			var oRoles = this.oContextsModel.getProperty("/values");
			if (this.oContextsModel.getProperty("/lastHitReached") === false) {
				var mConfig = {$skip: oRoles.length};
				return getData.call(this, mConfig, oRoles);
			}
			return Promise.resolve(oRoles);
		}
Checks if all data is loaded from back end.
If not, it retrieves the next chunk from the back end and then updates the model.
@returns {Promise} Resolves with additional data