at(index) {
        const { modelClass: ModelClass } = this;

        const rows = this._evaluate();
        if (index >= 0 && index < rows.length) {
            return new ModelClass(rows[index]);
        }

        return undefined;
    }
Returns the {@link Model} instance at index `index` in the {@link QuerySet} instance if
`withRefs` flag is set to `false`, or a reference to the plain JavaScript
object in the model state if `true`.

@param  {number} index - index of the model instance to get
@return {Model|undefined} a {@link Model} instance at index
                          `index` in the {@link QuerySet} instance,
                          or undefined if the index is out of bounds.