create vertex if not exists with nodejs gremlin

Question

I'm trying to use the technique described here for preventing duplicated edges in gremlin. I'm working in javascript and the query is failing with this error: Server error: Neither the map, sideEffects, nor path has a v-key: WhereEndStep(v) (500). This is the exact query I'm using:

import { process } from "gremlin";
const { statics } = process;
...
            g
            .V()
            .has('user', 'id', this.id)
            .fold()
            .coalesce(statics.unfold(), statics
                    .addV('user')
                    .property('id', this.id))
            .as('v')
            .V()
            .has('user', 'id', anotherUserId)
            .fold()
            .coalesce(statics.unfold(), statics
                    .addV('user')
                    .property('id', anotherUserId))
            .coalesce(statics
                    .inE('follow')
                    .where(
                        statics.outV().as('v')
                    ), statics.addE('follow').from_('v'))
            .V()
            .has('id', this.id)
            .outE('skip')
            .where(statics.inV().has('id', anotherUserId))
            .drop()
            .toList());

That's for completeness but the query never reaches the final outE('skip'), it fails at the last coalesce that's meant to prevent the duplicated edge. What could I be missing please?

Appears you are using the as() step and trying to reuse the defined labels after a reducing barrier step ( fold() in this case ). Labels do not persist across a fold(). You could try using aggregate() instead, just realize that aggregate creates a list that will need to be unfolded. — Taylor Riggan, Apr 13 '22 at 15:13
Thanks @TaylorRiggan, what would the `aggregate` query look like please? I'm new to gremlin. — Uche Ozoemena, Apr 13 '22 at 15:33

Uche Ozoemena · Accepted Answer · 2022-04-15T07:40:00.073

As someone pointed out in the comments, the problem was that labels do not persist across a .fold(). My solution was to break up the queries so I could preserve the relevant label. What worked was this:

import { process } from "gremlin";
const { statics } = process;
...
            // first make sure the second user is available
            g = g
            .V()
            .has('user', 'id', anotherUserId)
            .fold()
            .coalesce(statics.unfold(), statics
                    .addV('user')
                    .property('id', anotherUserId))

            // then add the edge after making sure the first user is available
            g
            .V()
            .has('user', 'id', this.id)
            .fold()
            .coalesce(statics.unfold(), statics
                    .addV('user')
                    .property('id', this.id))
            .as('v')
            .V()
            .has('user', 'id', anotherUserId)
            .coalesce(statics
                    .inE('follow')
                    .where(
                        statics.outV().as('v')
                    ), statics.addE('follow').from_('v'))
            .V()
            .has('id', this.id)
            .outE('skip')
            .where(statics.inV().has('id', anotherUserId))
            .drop()
            .toList());

create vertex if not exists with nodejs gremlin

1 Answers1