I don't think having a default direction applied for a non-directed edge is
a good idea; there wouldn't be a way to tell these edges apart later on.
I think it might be a better idea to just support non-directed edges.
John
On Mon, Jan 24, 2022 at 3:35 PM Jasper Blues <jas...@liberation-data.com>
wrote:
> Hi All,
>
> The quirk behind that CYPHER comes from Neo4j’s property graph model:
>
> All edges have a direction
> When direction is not relevant it can be ignored.
>
> This works will for read queries, for merge it is slightly quirky, however
> I believe the specification is reasonable:
>
> If we MERGE with an edge that does not specify a direction, it is because
> direction is irrelevant, just as in the read scenario
> Given this, the result is to intentionally assign a random direction
>
> I think the above behavior is OK. It would also be reasonable to pick a
> consistent direction, however this leads to potential compatibility issues:
>
> Users might start depending on an ‘implied’ direction
> When porting to/from Neo4j (interoperability is a strength - being able to
> attract users to the platform and have the users be confident they can
> migrate if ever they want aids adoption).
>
> So my 2c: Do what Neo4j does, and make it random, because the intention is
> “direction doesn’t matter”. However choosing a direction would also be ok.
> I don’t think rejecting the MERGE is great, because it differs from how
> other CYPHER graph DBs behave.
>
>
>
> > On Jan 25, 2022, at 7:06 AM, Josh Innis <josh.in...@bitnine.net> wrote:
> >
> > Hi All,
> >
> > The openCypher specification for MERGE has an ambiguous specification on
> > the subject of undirected relationships.
> >
> > Per the document on page 119 in the section titled "Merge on an
> undirected
> > relationship":
> >
> > MERGE can also be used with an undirected relationship. When it needs to
> > create a new one, it will pick a direction.
> >
> > Query:
> > MATCH (charlie:Person {name: 'Charlie Sheen'}), (oliver:Person {name:
> > 'Oliver Stone'})
> > MERGE (charlie)-[r:KNOWS]-(oliver)
> > RETURN r
> >
> > As 'Charlie Sheen' and 'Oliver Stone' do not know each other, this MERGE
> > query will create a KNOWS relationship between them. The direction of the
> > created relationship is arbitrary.
> >
> > We should probably clarify that. Having MERGE use undirected edges to
> find
> > paths is a potentially useful feature, but "The direction of the created
> > relationship is arbitrary" is unclear and should be clarified.
> >
> > I believe there are two potential ways to solve this issue:
> > Option 1: Do not let MERGE use undirected edges.
> > Option 2: Have a default direction that AGE will use every time MERGE
> > creates an edge where direction is not specified.
> >
> > Personally, I lean towards proposal 2 with the default direction being a
> > right directed edge. The other way limits functionality, and as long as
> the
> > decision we make is expressed well in the documentation, I don't believe
> it
> > is too confusing.
> >
> > Please let us know what you think.
>
>
