How to push values to property array Cypher-Neo4j

Question

I am new to Neo4j,I have two nodes user and files with a relationship :contains, the relationship has a property idwhich

Answer 1

In addition to the answer of jjaderberg in case that one of the attribute is null which could result in horrible errors:

SET n.id = coalesce(n.id, []) + n.additionalId

The coalesce goes through the comma seperated list (inside the brackets) from left to right and skips the variables that are Null values. So in this case if n.id is initially Null the coalesce would take the second parameter which is the empty array.