DELETE all relationships before creating new ones

General
1

We want to store a simple graph: (p:Person {id: 5)-[:LIKES]->(s:Song)

When we receive new information about the songs that a person likes, we want to

  • first delete existing LIKE-relationships for this person
  • create the new ones

The last part is easy:

foreach (song in $k.songdata |
    create (p)-[:LIKES]->(s:Song {title: k.title}))

For the first part we could use sth like

match (p:Person {id: k.id})-[:LIKES]->(s) detach delete (s)

But how can we combine these in one statement? We need to do this with plain Cypher, without apoc.

2

Hello @FrankR85 :slight_smile:

First, be aware it's a good thing to separate these queries, but here you have a way (moreover UNION should not be used like this):

MATCH (p:Person {id: k.id})-[r:LIKES]->(s)
DELETE (r)
UNION
FOREACH (song IN $k.songdata |
    CREATE (p)-[:LIKES]->(s:Song {title: k.title}))

You could also have a look at MERGE.

Regards,
Cobra

3

Thank you for your reply!
We would split the statement if we could, but we are using Neo4j streams and we noticed a massive degrade in throughput when we used apoc.cypher.runMany. It is up to 10x slower than a plain Cypher query.

That being said, I have some trouble getting your query to work within UNWIND.
Running

UNWIND $events as event
MATCH (p:Person {id: event.id})-[r:LIKES]->(s)
DELETE (r)
UNION
FOREACH (song IN event.songdata |
    CREATE (p)-[:LIKES]->(s:Song {title: k.title}))

yields an error because event is unknown in the clause following the union. I just can't get it to work. :see_no_evil:

If you know of another possibility to split the statements, we would be glad to hear it.

Unfortunatly we haven't figured out yet the exact reason why apoc.cypher.runMany is so slow.

4

Do you use a driver to send queries to your database?

UNWIND $events as event
MATCH (p:Person {id: event.id})-[r:LIKES]->(s)
DELETE (r)
UNION
UNWIND $events as event
FOREACH (song IN event.songdata |
    CREATE (p)-[:LIKES]->(s:Song {title: k.title}))
5

Thank you! This query works fine! :sunglasses:

I don't know what you mean with the question about the driver. We use a Cypher-Template as Neo4j streams sink ingestion strategy.

6

Nice:)

I was talking about this: