diff --git a/doc/doc-server/src/main/java/cn/axzo/nanopart/doc/entity/IndexNode.java b/doc/doc-server/src/main/java/cn/axzo/nanopart/doc/entity/IndexNode.java
index 906ac05e..7ddca53e 100644
--- a/doc/doc-server/src/main/java/cn/axzo/nanopart/doc/entity/IndexNode.java
+++ b/doc/doc-server/src/main/java/cn/axzo/nanopart/doc/entity/IndexNode.java
@@ -120,6 +120,7 @@ public class IndexNode extends BaseEntity<IndexNode> implements NodeValue, Index
 
 	// !! helper field
 
+	// to build a tree with TreeBuilder
 	@TableField(exist = false)
 	private boolean isValueRoot;