How to create unique CONSTRAINT to relationship by neo4j cypher?

Question

How to create unique CONSTRAINT to relationship by neo4j cypher?

Answer 1

If I understood your problem correctly, you want to enforce uniqueness of certain kind of relation rather than uniqueness of relation's certain attribute. If that's what you want, then you enforce such uniqueness by using "CREATE UNIQUE":

MATCH (root { name: 'root' })
CREATE UNIQUE (root)-[:LOVES]-(someone)
RETURN someone

The Neo4j Manual: Create unique relationships

Answer 2

it seems that a relationship constraint can only enforce the existence of a relationship property but not its uniqueness

CREATE CONSTRAINT ON ()-[like:LIKED]-() ASSERT exists(like.day)

http://neo4j.com/docs/developer-manual/current/cypher/#query-constraints-prop-exist-rels