How can I find and remove nodes for which two of a given relationship exist?

I have a requirement that in my database some nodes should have at most one relationship of a given type. For example, nodes with the label Heart should be connected via has to at most one node of label Body.

Here's some test data to illustrate the issue:

CREATE (h:Heart {animal: "zebra"})
CREATE (b1:Body {animal: "zebra"})
CREATE (b2:Body {animal: "elephant"})
CREATE (b1)-[:has]->(h)
CREATE (b2)-[:has]->(h)

CREATE (h2:Heart {animal: "mouse"})
CREATE (b4:Body {animal: "mouse"})
CREATE (b5:Body {animal: "leopard"})
CREATE (b4)-[:has]->(h2)
CREATE (b5)-[:has]->(h2)

With the following query I can find all nodes with connections which don't respect this rule:

MATCH (n:Heart) WHERE (:Body)-[:has]-(n)-[:has]-(:Body) RETURN n

I'd like though, to write a statement to remove the "extra" relationships. Right now I've got a query to remove the nodes composing these "extra" relationships, but it also removes the nodes I'd like to keep (one of each).

MATCH (b1:Body)-[:has]-(n)-[:has]-(b2:Body) DETACH DELETE b1, b2

This also doesn't group relationships by the center node, so I can't really remove all nodes except one, as it would not have the desired effect.

Does anyone know how can I write a statement that will remove all "extra" relationships until only one relationship has remains for each Heart node? Preferably the oldest one.

I don`t understand but if you need to remove relationships

MATCH (b1:Body)-[R1:has]-(n)-[R2:has]-(b2:Body) DELETE R1, R2

Ok....

IF (and this is a big if) I'm understanding this correctly, you don't want a zebra heart attached to an elephant body. If that's the case, then I think something like this might work...

MATCH (b1:Body)-[:has]-(h:Heart)-[:has]-(b2:Body)
WHERE b1.animal <> b2.animal
return n

This should provide a resultset where the animal properties of the 2 attached bodies are not equal.

Is this what you're looking for?????

The answers below help you in getting rid of the non-wanted relations.
Additional you might want to look into constraints; https://neo4j.com/docs/cypher-manual/current/administration/constraints/

Thank you everyone for your answer, but I think some of your didn't understand the issue properly.

The test data I wrote was only to illustrate the issue, it doesn't mean that's the only kind of data for which I want to implement such behavior.

I went with the following statement:

MATCH (:Body)-[r:has]->(h:Heart)
WITH h,COLLECT(r) AS rs
WHERE SIZE(rs)> 1
FOREACH (r IN rs[1..] |
    DELETE r
)

It was a an answer to the same question I also wrote on StackOverflow.

Again, thank you all for taking your time to answer this question and have a good week!

1 Like

Thanks for documenting it here as well!