Gremlin- unable to select 2 variables together when using coalesce

user_5644449 0
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I am using gremlin query language with Neptune gdb, and experienced a weird behavior when using select:

Let's say my graph is a single node

g.addV("test").property(id,"v1")

and I try this query:

g.V("v1").as("a")
.V().has("test","name","non-existing-name")
.fold().coalesce(unfold(),V("v1")).as("b")
.select("b")

The response is v[v1] as expected. If I do the same with select("a") at the end:

g.V("v1").as("a")
.V().has("test","name","non-existing-name")
.fold().coalesce(unfold(),V("v1")).as("b")
.select("a")

I get the same result, again- as expected.

the weird behavior is when I try to use select("a","b") at the end:

g.V("v1").as("a")
.V().has("test","name","non-existing-name")
.fold().coalesce(unfold(),V("v1")).as("b")
.select("a","b")

For some reason I get an empty response. Any idea why?

(I did find out that replacing the first as with store works, but I don't understand why)

amazon-neptune gremlin
1 Answers

I don't quite get the same results as you do for that second traversal and I would not expect to. Here is what I would expect to see:

gremlin> g.addV("test").property(id,"v1")
==>v[v1]
gremlin> g.V("v1").as("a").
......1>   V().has("test","name","non-existing-name").
......2>   fold().coalesce(unfold(),V("v1")).as("b").
......3>   select("b")
==>v[v1]
gremlin> g.V("v1").as("a").
......1>   V().has("test","name","non-existing-name").
......2>   fold().coalesce(unfold(),V("v1")).as("b").
......3>   select("a")
gremlin> g.V("v1").as("a").
......1>   V().has("test","name","non-existing-name").
......2>   fold().coalesce(unfold(),V("v1")).as("b").
......3>   select("a","b") 
gremlin>

Note that the last two traversal do not return results.

When you fold() you lose the path history to "a" so the traversal can't select() that step label any more. In general, you can't reference back to step labels that are on the opposite side of a reducing barrier step (like fold()). Other examples of reducing barriers would be steps like sum(), max(), min(), etc - where you have a number of traversers that reduce to a single one.

user_18317170
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