Add an edge failing after creating a vertex (Neptune InternalFailureException)

I am trying to create a user vertex and a city vertex (if they do not already exist in the graph), and then add an edge between the two of them. When I execute the command in the same traversal, I run into this InternalFailureException from Neptune.

g.V("user12345").
fold().
coalesce(unfold(),addV("user").property(id, "user-12345")).as('user').
V("city-ATL").
fold().
coalesce(unfold(), addV("city").property(id, "city-ATL")).as("city").
addE("lives_in").
  from("user").
  to("city")

{"code":"InternalFailureException","detailedMessage":"An unexpected error has occurred in Neptune.","requestId":"xxx"}

(Note in the case above, both user-12345 and city-ATL do not exist in the graph).

However, when I create the city before executing the command, it works just fine:

 gremlin> g.V("city-ATL").
          fold().
          coalesce(unfold(),       
                   addV("city").property(id, "city-ATL"))
 ==>v[city-ATL]

 gremlin> g.V("user-12345").
          fold().
          coalesce(unfold(),addV("user").property(id, "user-12345")).as('user').
          V("city-ATL").
          fold().
          coalesce(unfold(), addV("city").property(id, "city-ATL")).as("city").
          addE("lives_in").from("user").to("city")

 ==>e[1abd87d6-6f54-9e42-ae0a-47401c9dcfe6][user-12345-lives_in-city-ATL]

I am trying to build a traversal that can do them both together. Does anyone know why Neptune might be throwing this InternalFailureException when the city doesn't exist?

Solution

I will investigate further why you did not get a more useful error message but I can see that the Gremlin query will need to change. After a fold step any prior as labels are lost as fold reduces the traversers down to one. A fold is both a barrier and a map. You should be able to use store or aggregate(local) instead of as in this case where you have to use fold for each coalesce.

gremlin> g.V('user-1234').
......1>   fold().
......2>   coalesce(unfold(),addV('person').property(id,'user-1234')).store('a').
......3>   V('city-ATL').
......4>   fold().
......5>   coalesce(unfold(),addV('city').property(id,'city-ATL')).store('b').
......6>   addE('lives_in').
......7>     from(select('a').unfold()).
......8>     to(select('b').unfold())  

==>e[0][user-1234-lives_in->city-ATL]