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https://issues.apache.org/jira/browse/TINKERPOP-793?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15037770#comment-15037770
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stephen mallette commented on TINKERPOP-793:
--------------------------------------------
This issue has mostly been about {{select()}} {{by()}} but it also seems to
have some applicability to {{group().by()}}. I was writing some on Gremlin and
noticed this:
{code}
gremlin> g.V().hasLabel('person').group().by('name').by(out('develops'))
gremlin> g.V().hasLabel('person').group().by('name').by(out('uses'))
==>[daniel:v[10], matthias:v[10], stephen:v[10], marko:v[11]]
{code}
[~dkuppitz] was quick to inform me that since {{out('develops')}} doesn't
return anything for "daniel" in The Crew, the whole traversal returns nothing -
so i have to do:
{code}
gremlin> g.V().hasLabel("person").group().by("name").by(out("develops").fold())
==>[daniel:[], matthias:[v[10]], stephen:[v[10], v[11]], marko:[v[10], v[11]]]
{code}
i think this generally means that I should (almost) always add {{fold()}} to
inner traversals as it seems like the most safe way to get at what i intend.
if it had thrown a {{FastNoSuchElementException}} it might have been hinted at
what was wrong perhaps and then I might have realized I needed a {{fold}} or
something, but I'm not sure new users would jump to that conclusion. It's also
troubling that this issue might not be as easy to diagnose on our end as
"iterate your traversal" so when the mailing list has these kinds of issues we
might spend a lot of time trying to solve them.
anyway, i don't think i'll get caught out by this issue again as I'll know what
I'm looking for and I'll be sprinkling {{fold()}} in a bunch of places. I'll
probably get bored of sprinkling at some point and write myself a
{{AllowNonProductiveInnerTraversalStrategy}} to auto-append my {{fold()}} steps
for me. :)
> select should stop traversal on unproductive by
> -----------------------------------------------
>
> Key: TINKERPOP-793
> URL: https://issues.apache.org/jira/browse/TINKERPOP-793
> Project: TinkerPop
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.0.2-incubating
> Reporter: Matt Frantz
> Labels: breaking
>
> Instead of throwing an exception, we should stop traversal if a {{by}} clause
> fails to produce.
> Current:
> {noformat}
> gremlin> g.V().as('a').out().out().as('b')
> .select('a', 'b')
> .by('name')
> .by(has('name', 'lop').values('name'))
> The provided start does not map to a value: v[5]->[HasStep([name.eq(lop)])]
> {noformat}
> Desired:
> {noformat}
> gremlin> g.V().as('a').out().out().as('b')
> .select('a', 'b')
> .by('name')
> .by(has('name', 'lop').values('name'))
> ==>[a:marko, b:lop]
> {noformat}
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