Cypher: return whether an optional relationship exists?

[Aseem Kishore]

I want to execute a Cypher query that'll return every user in our graph, along with whether I follow them or not.

So I first tried a query like this:

START me=node(1), user=[all users] # index query omitted

MATCH (me) -[r?:follows]-> (user)

RETURN user, r?

My intent with the return `r?` was thinking that the `?` operator would evaluate to true or false, like in CoffeeScript. ("Does r exist?") I realize now that's not the case.

Turns out that this was a syntax error; just returning `r` handles the optional case. But it does something I don't want: in cases that match, it returns the whole relationship JSON.

In this case, I just want to know whether the relationship exists or not. How can I do that?

I thought I'd try `r IS NULL` or `r ISNT NULL`, but those are also syntax errors. I guess `IS NULL` etc. is only accepted in the WHERE clause.

I then thought to try returning TYPE(r), so it'd be "follows" or <null> (hopefully). But this fails silently. (BUG)

Finally, I tried returning COUNT(r), which returns 1 or 0, and that works! So I've found a decent option.

But that does feel a bit hacky to me, so I thought I'd ask if there's a better/proper way of just returning true/false. Thanks!

Aseem

[Andres Taylor]

You can do it by just returning r. If it's null - you are not following the user. If it's non-null - you follow.

We'll have a chat about the question mark, and allowing predicates in the return clause, and see if we need to change anything.

Thanks for bringing this up!

Andrés

[Aseem Kishore]

Hey Andres,

So sorry for the delayed reply here. Just ran into this same issue again and had to dig through my email to remember how I worked around this.

You're right that I can just return r, but I ran into this in the first place because I didn't want to return the entire relationship JSON for *each* relationship. In this case, all I care about is whether the relationship exists.

It'd be nice if I could thus just return `true` or `false` for each relationship.

Cheers,

Aseem

[Andres Taylor]

On Thu, Mar 1, 2012 at 6:29 AM, Aseem Kishore <aseem....@gmail.com> wrote:

<snip>

It'd be nice if I could thus just return `true` or `false` for each relationship.

<snip>

Yeah, I agree. I've been thinking of adding predicates as a normal expression, which would let you do something like:

RETURN r IS NULL 

and that would give you exactly what you want.

We'll get there. Thanks for your patience.

Andrés

[Aseem Kishore]

No problem, and great to hear! Thanks for the continuous improvements. =)