Neo4j: Cypher - Creating relationships between a collection of nodes / Invalid input '[':
When working with graphs we’ll frequently find ourselves wanting to create relationships between collections of nodes.
A common example of this would be creating a linked list of days so that we can quickly traverse across a time tree. Let’s say we start with just 3 days:
MERGE (day1:Day {day:1 })
MERGE (day2:Day {day:2 })
MERGE (day3:Day {day:3 })
RETURN day1, day2, day3And we want to create a 'NEXT' relationship between adjacent days:
(day1)-[:NEXT]->(day2)-[:NEXT]->(day3)The most obvious way to do this would be to collect the days into an ordered collection and iterate over them using http://docs.neo4j.org/chunked/stable/query-foreach.html, creating a relationship between adjacent nodes:
MATCH (day:Day)
WITH day
ORDER BY day.day
WITH COLLECT(day) AS days
FOREACH(i in RANGE(0, length(days)-2) |
CREATE UNIQUE (days[i])-[:NEXT]->(days[i+1]))Unfortunately this isn’t valid syntax:
Invalid input '[': expected an identifier character, node labels, a property map, whitespace, ')' or a relationship pattern (line 6, column 32)
" CREATE UNIQUE (days[i])-[:NEXT]->(days[i+1]))"
^It doesn’t seem to like us using array indices where we specify the node identifier.
However, we can work around that by putting days and days into single item arrays and using nested FOREACH loops on those, something Michael Hunger showed me last year and I forgot all about!
MATCH (day:Day)
WITH day
ORDER BY day.day
WITH COLLECT(day) AS days
FOREACH(i in RANGE(0, length(days)-2) |
FOREACH(day1 in [days[i]] |
FOREACH(day2 in [days[i+1]] |
CREATE UNIQUE (day1)-[:NEXT]->(day2))))Now if we do a query to get back all the days we’ll see they’re connected:
About the author
I'm currently working on short form content at ClickHouse. I publish short 5 minute videos showing how to solve data problems on YouTube @LearnDataWithMark. I previously worked on graph analytics at Neo4j, where I also co-authored the O'Reilly Graph Algorithms Book with Amy Hodler.