add support for merging virtual nodes / rels

[tomasonjo]

for now we can only create virtual nodes and it would be very helpyful if we could merge virtual nodes so that one can easily create a virtual graph.

CALL apoc.merge.vNode(['Label'], {key:value,…​}) YIELD node

Example use:

MATCH (h1:Hero)-[:KNOWS]->(h2:Hero)
WHERE h1.louvain <> h2.louvain
WITH h1.louvain as first,h2.louvain as second,count(*) as weight
call apoc.cypher.run("match (:`Hero`{louvain:"+first+"}) return count(*) as count", null) yield value 
CALL apoc.merge.vNode(['Cluster'], {community:first,size: value.count}) YIELD node as nodeFrom
CALL apoc.merge.vNode(['Cluster'], {community:second}) YIELD node as nodeTo
CALL apoc.create.vRelationship(nodeFrom,'COMMON',{weight:weight}, nodeTo) YIELD rel
RETURN nodeFrom,nodeTo,rel

Not sure if we want one parameter for merging only and one for ON CREATE SET,ON MATCH SET. In my example I don't need it

[vga91]

The above example with apoc.create.vNode instead of apoc.merge.vNode generates a correct virtual relationship.
Moveover, we can currently use the apoc.create.clonePathToVirtual and apoc.create.clonePathsToVirtual procedures to create virtual paths: https://neo4j.com/labs/apoc/4.3/overview/apoc.create/apoc.create.clonePathsToVirtual/ .
And to date, it looks more like a function regarding GDS.
I'm closing it, please reopen it if needed