function getId(value) {
        if (angular.isObject(value) && !angular.isArray(value)) {
          return 'object_' + (value.$$mdSelectId || (value.$$mdSelectId = ++selectNextId));
        }
        return value + '';
      }
If the value is an object, get the unique, incremental id of the value.
If it's not an object, the value will be converted to a string and then returned.
@param value
@returns {string}