OnDocumentWriting.cs

protected override void OnDocumentWriting(DocumentWritingEventArgs docArgs)
{
    var currentNode = _nodeFactoryFacade.GetNode(docArgs.NodeId);

    var categories = currentNode.GetProperty("categories").Value;
    if (!string.IsNullOrEmpty(categories))
    {
        var categoryNodeIdsXml = XElement.Parse(categories);
        var categoryNodeIds = categoryNodeIdsXml.Descendants("nodeId");
        foreach (var categoryNodeId in categoryNodeIds)
        {
            docArgs.Document.Add(new Field("categories", categoryNodeId.Value, Field.Store.YES, Field.Index.ANALYZED));
        }
    }


    base.OnDocumentWriting(docArgs);
}

OnGatheringNodeData.cs

protected override void OnGatheringNodeData(IndexingNodeDataEventArgs e)
{
    var currentNode = _nodeFactoryFacade.GetNode(e.NodeId);
    var categories = e.Node.Descendants("XPathCheckBoxList").Descendants("nodeId");
    e.Fields.Add("categories", string.Join("|", categories.Select(x=>x.Value)));
}