Gremlin - only add a vertex if it doesn't exist - groovy

Question

I have an array of usernames (eg. ['abc','def','ghi']) to be added under 'user' label in the graph.
Now I first want to check if the username already exists (g.V().hasLabel('user').has('username','def')) and then add only those for which the username property doesn't match under 'user' label.
Also, can this be done in a single gremlin query or groovy script?
I am using titan graph database, tinkerpop3 and gremlin REST server.

Answer 1

With "scripts" you can always pass a multi-line/command script to the server for processing to get what you want done. This question is then answered with normal programming techniques using variables, if/then statements, etc:
t = g.V().has('person','name','bill')
t.hasNext() ? t.next() : g.addV('person').property('name','bill').next()
or perhaps:
g.V().has('person','name','bill').tryNext().orElseGet{
g.addV('person').property('name','bill').next()}
But these are groovy scripts and ultimately TinkerPop recommends avoiding scripts and closures in favor of a pure traversal. The general way to handle a "get or create" in a single traversal is to do something like this:
gremlin> g.V().has('person','name','bill').fold().
......1> coalesce(unfold(),
......2> addV('person').property('name','bill'))
==>v[18]
Also see this StackOverflow question for more information on upsert/"get or create" patterns.

Answer 2

You can do it directly using :
g.V().has('user','username','def').fold().coalesce(unfold(),addV('user').property('username','def'))

Answer 3

Just adding into this answer. It is better to use the following idempotent query. Coalesce works like an if-else statement. Refer to, https://spin.atomicobject.com/2021/08/10/idempotent-queries-in-gremlin/ for more information. Also, if you are noticing that the entry is not being saved, make sure you commit the changes using .next().
g.V().hasLabel('user').has('username','def')
.fold()
.coalesce(
__.unfold(),
__.addV('user').property('username','def')
)
.next()