CODEWITHSUNDEEP
neo4jcypher
Rowan Smith
Rowan Smith

Reputation: 2180

Neo4j stop searching an undirected path when given node is encountered

I have the following test data in Neo4j:

merge (n1:device {name:"n1"})-[:phys {name:"phys"}]->(:interface {name:"n1a"})-[:cable {name:"cable"}]->(:interface {name:"n2a"})-[:phys {name:"phys"}]->(n2:device {name:"n2"})
merge (n1)-[:phys {name:"phys"}]->(:interface {name:"n1b"})-[:cable {name:"cable"}]->(:interface {name:"n2b"})-[:phys {name:"phys"}]->(n2)
merge (n1)-[:phys {name:"phys"}]->(:interface {name:"n1c"})-[:cable {name:"cable"}]->(:interface {name:"n2c"})-[:phys {name:"phys"}]->(n2)
merge (n1)-[:phys {name:"phys"}]->(:interface {name:"n1d"})-[:cable {name:"cable"}]->(:interface {name:"n2d"})

Giving:

enter image description here

While this example has exactly 3 relationships and 2 nodes on each of the 4 paths between each of n1 and n2, my real data could have many more, and also many more paths.

This is a undirected graph and in the real dataset, relationships on parts of each path are in either direction.

I know that every path starts at a :device and either just ends at a non :device or ends at a :device, and along the way there could be any number of relationships and other non :device nodes.

So I am looking to do:

match p=(:device {name:"n1"})-[*]-(:device) return (p)

and have it return the same, (I would be happy with double), number of records as:

match p=(:device {name:"n1"})-[*]->(:device) return (p)

So I am looking for a way to stop matching relationships and cease following the path when the first (:device) is encountered in the path.

From my limited understanding, I could easily achieve this by making every relationship bidirectional. However I have avoided that option to date as I have read it is bad practice.

Extra for experts :-)

Additionally, I would like a way to return any full paths that don't end at a :device (eg, the bottom one)

Thanks

Upvotes: 0

Views: 156

Answers (1)

InverseFalcon
InverseFalcon

Reputation: 30407

This is a use case that is a little hard to do with just Cypher, as we don't have a way to specify "follow a variable-length path and stop when you reach another node of this type".

We can do something like this when we use LIMIT, but that becomes too restrictive when we don't know how many results there will be, or we need to do this for multiple starting nodes.

Because of this, there are some APOC path finder procedures that include more flexible options. One of these is a labelFilter option which lets you describe how to filter nodes with particular labels found during expansion (blacklisting, whitelisting, etc). One of these filters is called a termination filter (uses an / symbol before the appropriate label), which means to include the path to that node as a result, and stop expansion, which is exactly what you're looking for.

After you install APOC, you can use the apoc.path.expandConfig() procedure, starting from your start node, and supply the labelFilter config parameter to get this behavior:

MATCH (start:device {name:"n1"})
CALL apoc.path.expandConfig(start, {labelFilter:'/device'}) YIELD path
RETURN path

Upvotes: 3

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