Node is deleted and cannot be used to create a relationship

[marcel_olij]

I have a node-link model ( which is a graph ) of an electricity grid in Neo4j, so this should fit like a glove.

The nodes and links are objects of their own, which are traversable in an abstract manner.

Link has a starting node (:Link) – [:begint_bij] –> (:Node) and an ending node (:Link) – [:eindigt_bij] –> (:Node)

Every :Node and :Link has references to one or more objects. Links and Nodes are uniquely defined by their references in this model.

The model is generated by the object-oriented GIS model and because of the generating method ( which I have no influence on ) redundant Node Link (pairs) are in the dataset.

In the picture we see a cloud of Nodes ( purple ) and Links ( green ) surrounding an electricity cable.

In the detail you can see the redundant pairs clearly with a pattern:

A Node-Link pair where the Node has only relationship to ( cables, gisreferences and links ) and the link has only relationships to ( cables, gisreferences and nodes )

For removing the redundant Node-Link pairs I have come up with this query:

call apoc.periodic.iterate (

" MATCH (fld:e_lv_function_cable ) where not exists (fld.done) Return fld",

" match (fld) - [:GenObject] - (fn:Node)

match (fn) - [:begint_bij] - (l1:Link)

with [(fn) - - (obj1) | labels(obj1)] AS conn1, [(l1) - - (obj2) | labels(obj2)] AS conn2, fn, l1, fld

where all( lb1 IN conn1 where lb1[0] in ['e_lv_function_cable','GisReference','Link'] )

and all( lb2 IN conn2 where lb2[0] in ['e_lv_function_cable','GisReference','Node'] )

with l1, fn, fld

match (l1) - [:eindigt_bij] - (n2)

match (l2:Link) - [r] - (fn) - [:begint_bij] - (l1)

Call apoc.do.when (

n2 is not null and l2 is not null and r is not null,

'Call apoc.create.relationship (l2, type(r), {}, n2) YIELD rel Detach Delete l1, fn',

'',

{n2: n2, l2: l2, r: r, l1: l1, fn: fn}) YIELD value

with fld

set fld.done=true

", {batchSize:100, parallel:false}) yield batches, total return batches, total

And it does the job.

However in a few cases I get an error ( in the debug log 😞

Failed to invoke procedure apoc.do.when: Caused by: org.neo4j.graphdb.NotFoundException: Node is deleted and cannot be used to create a relationship

And sometimes:

Failed to invoke procedure apoc.do.when: Caused by: java.lang.IllegalStateException: Kernel API returned non-existent relationship type: -1

And it almost makes sense to me.

Can anyone please:

• (better) explain to me what is causing this, and
• Point me t a query that does the job 100%

Thx.

[Cobra]

Hello @marcel.olij and welcome to the Neo4j community

When you load the nodes and relationships, did you use MERGE?

• merge nodes
• merge relationships
• creating nodes from a list parameter

Regards,
Cobra

[marcel_olij]

Cobra,

The redundant nodes are in the input and can't be merged on basis of their ids or rels.

BR,

Marcel

[Cobra]

Hello @marcel.olij

Don't you use unique constraints to create your database?

Regards,
Cobra

[marcel_olij]

Cobra,

The nodes and links are themselves unigue in the input, so unique constraints would not do the job.
The software that creates the node link model has a reason to generate these extra node-link pairs ( I don't see what it is though ). They are of no use to me and I want them out for better perfomance and model hygiëne.
If i could use a unique constraint, I still must be able to connect the node-links at import again and that would probably harder than removing these pairs after creation.

BR,

Marcel

BR,

Marcel