Cypher conditional create and return values.

[Aran Mulholland]

I have a couple of nodes. A user and a post. I want to add a like relationship between the user and the post (user-[LIKE]->post) only if the like does not already exist. This query does the trick

START u=node:node_auto_index(UserIdentifier = 'USER12'), p=node:node_auto_index(PostIdentifier = 'USER5POST4')

MATCH u-[existingLike?:LIKE]->p

WHERE existingLike IS NULL

CREATE u-[l:LIKE]->p

RETURN u, l, p;

In my service call I want to know if the user exists, the post exists and if the new like has been added. This part works only OK when the new like is added as I get (u, l and p) return values. When there is an existing like or the user or post is missing I get no return values.

Is there a way to return u and p using only one Cypher query whether or not the create statement executed?

[Peter Neubauer]

Would MERGE be what you are looking for?

http://docs.neo4j.org/chunked/snapshot/query-merge.html#merge-merge-with-on-create-and-on-match

Cheers,

/peter neubauer

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[Michael Hunger]

For neo4j 1.9 see 

create unique

Sent from mobile device

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[Aran Mulholland]

Thanks Peter, I'll check out merge. 

Michael if the relationship has a time stamp property (which on further thought I'd like to add) then CreatUnique will create a new one every time. Is there anyway to use CreateUnique in this scenario?

Sent from my iPhone

[Wes Freeman]

Like this (assuming you always want to update the timestamp):

START u=node:node_auto_index(UserIdentifier = 'USER12'), p=node:node_auto_index(PostIdentifier = 'USER5POST4')

create unique u-[l:LIKE]->p
set l.timestamp = timestamp()

Wes

[Aran Mulholland]

Thanks Wes, but I don't want to update the time stamp. I only want to create if the relationship does not exist.

[Michael Hunger]

You can filter with WHERE,

START u=node:node_auto_index(UserIdentifier = 'USER12'), p=node:node_auto_index(PostIdentifier = 'USER5POST4')

create unique u-[l:LIKE]->p

       WHERE not ( has (l.timestamp))

set l.timestamp = timestamp()

If you want to return l you have to do some collection magic:

START u=node:node_auto_index(UserIdentifier = 'USER12'), p=node:node_auto_index(PostIdentifier = 'USER5POST4')

create unique u-[l:LIKE]->p

       WITH collect(l) as rels

       FOREACH (r2 in filter(r1 in rels : not (has (l.timestamp))) r2.timestamp = timestamp())

       RETURN head(rels) as l

If you already use 2.0 you will be able to use MERGE with ON CREATE, but it is not done yet, meaning that the current M03 only supports merge for single node patterns not full patterns.

if you still want to return l, you might need  use use CASE WHEN in 2.0

Michael

[Brian Gates]

I'm trying to accomplish something similar, using 2.0.  http://stackoverflow.com/questions/17661543/neo4j-2-0-cypher-enforce-unique-username-create-unique-node-or-fail