I'm getting an unexpected StopIteration error with some gremlin queries that contain a count step within nested filter steps.
This error can be recreated with the following code (using Gremlin-Python, 3.5.0 in my case):
filter_header = g.addV().id().next()
count_headers = [g.addV().id().next() for _ in range(10)]
for i, c in enumerate(count_headers):
# Add 10 nodes
sub_nodes = [g.addV().id().next() for _ in range(10)]
# Connect them all to the header
for s in sub_nodes:
g.V(c).addE('edge').to(__.V(s)).iterate()
# Connect i of them to the filter header
for s in sub_nodes[:i]:
g.V(filter_header).addE('edge').to(__.V(s)).iterate()
# This raises StopIterationError
g.V(count_headers).filter(
__.out('edge').filter(
__.in_('edge').hasId(filter_header)
).count().is_(P.gt(1))
).count().next()
(Equivalently if using toList instead of next I get an empty list)
However this error doesn't happen if you unfold after the count:
# No StopIterationError
g.V(count_headers).filter(
__.out('edge').filter(
__.in_('edge').hasId(filter_header)
).count().unfold().is_(P.gt(1))
).count().next()
Neither does it happen if you use map instead of filter:
# No StopIterationError
g.V(count_headers).as_('c').map(
__.out('edge').filter(
__.in_('edge').hasId(filter_header)
).count().is_(P.gt(1))
).select('c').count().next()
I've tested and this error doesn't happen when using TinkerGraph, so I suspect this is specific to AWS Neptune.
I'd really appreciate any guidance as to why this happens, if I'm doing anything wrong, or what the differences are that means this just happens in Neptune. Alternatively - if the consensus is that this is a bug - I'd appreciate it if anyone could let me know where to raise it.
For anyone that finds themselves here: this was a bug that was fixed in Neptune Engine release 1.1.1.0.
"Fixed a rare Gremlin bug where no results were returned when using nested filter() and count() steps in combination"
(Thanks to the Neptune team for fixing!)