CODEWITHSUNDEEP
groovygraph-databasesgremlintitantinkerpop3
Vipul Sharma
Vipul Sharma

Reputation: 798

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

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.

Upvotes: 14

Views: 12649

Answers (3)

stephen mallette
stephen mallette

Reputation: 46226

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.

UPDATE: As of TinkerPop 3.6.0, the fold()/coalesce()/unfold() pattern has been largely replaced by the new steps of mergeV() and mergeE() which greatly simplify the Gremlin required to do an upsert-like operation. Under 3.6.0 and newer versions, you would write:

g.mergeV([(label): 'person', name: 'bill'])

Upvotes: 42

deirdreamuel
deirdreamuel

Reputation: 857

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()

Upvotes: 0

Hossam Kandil
Hossam Kandil

Reputation: 26

You can do it directly using :

g.V().has('user','username','def').fold().coalesce(unfold(),addV('user').property('username','def'))

Upvotes: 1

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