Using SDN4， I can save nodes(using NodeEntity) and relationships( using RelationshipEntity).
When I upgraded to SDN46， I can only save nodes(using Node), How can Ioperate the relationship alone？
Relationships / RelationshipEntities are not an entity anymore starting with SDN 6. This representation was not accurate before, because relationships cannot exist without the a start and an end node. In the context of an object mapper those start and end nodes are explicitly defined entities/nodes.
If you really want to operate on the relationships on their own, you would have to use custom Cypher queries e.g. in combination with the Neo4jClient.