Why does my Gremlin traversal add only one edge?

As described in another question, I am attempting to add several “identity” vertices into a “group” vertex. Based on the recipe recommendation, I’m trying to write the traversal steps in such a way that the traversers iterate the identity vertices instead of appending extra steps in a loop. Here’s what I have:

gts.V(group)
  .addE('includes')
  .to(V(identityIds as Object[]))
  .count().next()

This always returns a value of 1, no matter how many IDs I pass in identityIds, and only a single edge is created. The profile indicates that only a single traverser is created for the __.V even though I’m passing multiple values:

Traversal Metrics
Step                                                               Count  Traversers       Time (ms)    % Dur
=============================================================================================================
TinkerGraphStep(vertex,[849e1878-86ad-491e-b9f9...                     1           1           0.633    40.89
AddEdgeStep({label=[Includes], ~to=[[TinkerGrap...                     1           1           0.915    59.11
  TinkerGraphStep(vertex,[50d9bb4f-ed0d-493d-bf...                     1           1           0.135
                                            >TOTAL                     -           -           1.548        -

Why is only a single edge added to the first vertex?

Answer

The to() syntax you are using is not quite right. A modulator like to() expects the traversal you provide it to produce a single Vertex not a list. So, given V(identityIds) only the first vertex returned from that list of ids will be used to construct the edge. Step modulators like to(), by(), etc. tend to work like that.

You would want to reverse your approach to:

gts.V(identityIds)
  .addE('includes')
  .from(V(group))
  .count().next()

But perhaps that leads back to your other question.