Cypher: Nullpointer with 2 NOTs in the where clause

[Luanne Coutinho]

Hi,

Is something wrong with this query or is it an issue with Cypher (1.7)-

The following query produces an NPE whether or not each NOT is grouped by parentheses- a single NOT works fine:

start n=node(1)

match (person)<-[:knows|works_with]-(n)-[?:friend]-(friend)-[?:knows|works_with]-(friendProfile)

where (not((person)-[:blocks]-(n))) and (not((friendProfile)-[:blocks]-(n)))

return person.name,friendProfile.name

java.lang.NullPointerException

at org.neo4j.cypher.internal.commands.HasRelationshipTo.isMatch(Predicate.scala:113)

at org.neo4j.cypher.internal.commands.Not.isMatch(Predicate.scala:94)

at org.neo4j.cypher.internal.commands.And.isMatch(Predicate.scala:72)

at org.neo4j.cypher.internal.pipes.FilterPipe$$anonfun$1.apply(FilterPipe.scala:31)

at org.neo4j.cypher.internal.pipes.FilterPipe$$anonfun$1.apply(FilterPipe.scala:31)

at scala.collection.TraversableLike$$anonfun$filter$1.apply(TraversableLike.scala:213)

at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)

at scala.collection.immutable.List.foreach(List.scala:45)

at scala.collection.TraversableLike$class.filter(TraversableLike.scala:212)

at scala.collection.immutable.List.filter(List.scala:45)

at org.neo4j.cypher.internal.pipes.FilterPipe.createResults(FilterPipe.scala:31)

at org.neo4j.cypher.internal.pipes.ExtractPipe.createResults(ExtractPipe.scala:38)

at org.neo4j.cypher.internal.pipes.ColumnFilterPipe.createResults(ColumnFilterPipe.scala:37)

at org.neo4j.cypher.internal.executionplan.ExecutionPlanImpl$$anonfun$3$$anonfun$apply$1.apply(ExecutionPlanImpl.scala:62)

at org.neo4j.cypher.internal.executionplan.ExecutionPlanImpl$$anonfun$3$$anonfun$apply$1.apply(ExecutionPlanImpl.scala:62)

at org.neo4j.cypher.PipeExecutionResult.immutableResult(PipeExecutionResult.scala:36)

at org.neo4j.cypher.PipeExecutionResult.iterator(PipeExecutionResult.scala:138)

at org.neo4j.cypher.PipeExecutionResult.hasNext(PipeExecutionResult.scala:140)

at scala.collection.Iterator$$anon$19.hasNext(Iterator.scala:334)

at scala.collection.JavaConversions$IteratorWrapper.hasNext(JavaConversions.scala:562)

Thanks

Luanne

[Luanne Coutinho]

Ignore this for a bit...

[Luanne Coutinho]

Might have been the missing space between not and the starting parenthesis...

[Luanne Coutinho]

Okay sorry for all the mails, I'm a bit confused. My earlier queries worked fine without the space (with a single not). Trying to figure out the conditions under which this fails.

[Luanne Misquiita]

So I definitely get this exception with the query. The space between the not and the parenthesis does not matter. Any ideas why?

Sent from my iPad

[Michael Hunger]

Most probably due to the optional relationships, you might get null nodes in between.

coalesce doesn't work for nodes (except if you pass in something like the ref-node).

Could you try the ! modifier, which would probably cause the expression to evaluate to false if the node is null.

Cheers

Michael

Would you be able to set up a mini-graph in the neo4j-console, that exhibits the problem? http://console.neo4j.org?init=(0) gives you an empty one to start with or use console.neo4j.org with the matrix graph.

You can use mutating cypher or geoff to build up your graph.

[Luanne Coutinho]

Hi Michael,

Must be the optional path, but couldn't use the ! since it appears to work only on properties?

In any case, I created a sample of the graph at http://tinyurl.com/6lgcqab

Here is the part of the query that works:

start n=node(1)

match (person)<-[:knows|works_with]-(n)-[?:friend]-(friend)-[?:knows|works_with]-(friendProfile)

where (not((person)-[:blocks]-(n)))

return person.name,friendProfile.name

Adding the second not fails:

start n=node(1)

match (person)<-[:knows|works_with]-(n)-[?:friend]-(friend)-[?:knows|works_with]-(friendProfile)

where (not((person)-[:blocks]-(n))) and (not((friendProfile)-[:blocks]-(n)))

return person.name,friendProfile.name

I now realize with this and the previous question about transposing rows that what I'm trying to do is a union. In any case, please let me know if there's a better way to write this query or get it working, and whether I should just split into two separate queries.

Thanks

Luanne

[Andres Taylor]

On Tue, May 1, 2012 at 4:12 PM, Luanne Coutinho <luanne....@gmail.com> wrote:

Hi Michael,

Must be the optional path, but couldn't use the ! since it appears to work only on properties?

In any case, I created a sample of the graph at http://tinyurl.com/6lgcqab

Here is the part of the query that works:

start n=node(1)

match (person)<-[:knows|works_with]-(n)-[?:friend]-(friend)-[?:knows|works_with]-(friendProfile)

where (not((person)-[:blocks]-(n)))

return person.name,friendProfile.name

Adding the second not fails:

start n=node(1)

match (person)<-[:knows|works_with]-(n)-[?:friend]-(friend)-[?:knows|works_with]-(friendProfile)

where (not((person)-[:blocks]-(n))) and (not((friendProfile)-[:blocks]-(n)))

return person.name,friendProfile.name

I now realize with this and the previous question about transposing rows that what I'm trying to do is a union. In any case, please let me know if there's a better way to write this query or get it working, and whether I should just split into two separate queries.

Nice work Luanne. I couldn't re-produce the problem with the query someone else supplied, but with your graph set up and your query, it was super easy to locate and fix. Lovely!

https://github.com/neo4j/community/pull/484

Andrés

[Luanne Coutinho]

Andres, that's great, thanks. Till the fix is available, I've split into two queries.

Regards
Luanne

[Andres Taylor]

On Wed, May 2, 2012 at 11:37 AM, Luanne Coutinho <luanne....@gmail.com> wrote:

Andres, that's great, thanks. Till the fix is available, I've split into two queries.

It had nothing to do with the number of comparisons - just make sure to check for null before you check the relationships. Or, wait for the PR to be merged in :)

Andrés 

[Luanne Coutinho]

In this particular query, is it possible to check for a null before checking the relationship?
By splitting the query into two, I got rid of the optional part so that query will either return data or not.

-Luanne

[Michael Hunger]

not (node is null) AND node-[:rel]->other

??

Michael

[Andres Taylor]

On Wed, May 2, 2012 at 12:21 PM, Michael Hunger <michael...@neotechnology.com> wrote:

not (node is null) AND node-[:rel]->other

That's it, yes.

Andrés 

[Luanne Coutinho]

Perfect, thanks!