I’ve written before about using the cypher CREATE statement to add inferred information to a neo4j graph and sometimes we want to do that but have to deal with optional properties while creating our new relationships.

For example let’s say we have the following people in our graph with the ‘started’ and ‘left’ properties representing their tenure at a company:

CREATE (person1 { personId: 1, started: 1361708546 })
CREATE (person2 { personId: 2, started: 1361708546, left: 1371708646 })
CREATE (company { companyId: 1 })

We want to create a ‘TENURE’ link from them to the company including the ‘started’ and ‘left’ properties when applicable and might start with the following query:

START person = node:node_auto_index('personId:1 personId:2'), 
      company = node:node_auto_index('companyId:1') 
CREATE person-[:TENURE_AT { started: person.started, left: person.left }]-company 
RETURN person, company

which throws an exception because not all people have a ‘left’ property:

Error: org.neo4j.cypher.EntityNotFoundException: The property 'left' does not exist on Node[1]

We tweak our query a bit to make the property optional:

START person = node:node_auto_index('personId:1 personId:2'), 
      company = node:node_auto_index('companyId:1') 
CREATE person-[:TENURE_AT { started: person.started, left: person.left? }]-company 
RETURN person, company

which still doesn’t work:

Error: java.lang.IllegalArgumentException: Null parameter, key=left, value=null

After looking at this for a while Alistair pointed out that we should just split the updating of the optional property from the creation of the relationship so we end up with the following:

START person = node:node_auto_index('personId:1 personId:2'), 
      company = node:node_auto_index('companyId:1') 
CREATE person-[tenure:TENURE_AT { started: person.started }]-company 
WITH person, tenure, company
WHERE HAS(person.left)
SET tenure.left = person.left 
RETURN person, company