Converting a star-topology to a linked list in Neo4j - linked-list

Question

When I initially created my data set I thought a star topology would be nice, but now that I've gotten some sample data, I'd rather be using a linked list topology. Now I want to do an in-place conversion using Cypher only, and in one call. Is this possible?
To use a simplified example: a Blog node can have several children Post nodes, so I attached them directly like this:
CREATE (a:Blog), (a)-[:HAS]->(b:Post), (a)-[:HAS]->(c:Post)
RETURN a,b,c
Now as my design moves along, I think I'd rather have this as a linked list, something like this (assuming the :FIRST post is already created):
MATCH (a:Blog)-[r:FIRST]->(b:Post)
CREATE (a)-[:FIRST]->(c:Post)-[:PREV]->(b)
DELETE r
RETURN a,b,c
So I can code the linked list fine. What I need to do is to convert the old star topologies to the new linked list format. I tried a bunch of things but nothing worked, so here is my best guess, but is completely non-functional code:
MATCH (b:Blog)-[:HAS]-(p:Post)
WITH b, collect(p) as posts
CREATE b-[:FIRST]->posts[0]
FOREACH( i IN range(1,len(posts)-1) |
CREATE posts[i]-[:PREV]->posts[i-1]
)
RETURN b,p
Indexing collections doesn't seem to work in a CREATE statement (neither inside or outside the FOREACH).
Any way to achieve this translation?
EDIT
I thought an additional way might be to tag the posts with ordinal values and do some iteration over sequential ordinals, but I get a syntax error trying to add the ordinals:
MATCH (b:Blog)-[:HAS]-(p:Post)
WITH b, collect(p) as posts
FOREACH( i in range(0,length(posts)) |
SET posts[i].ordinal = i
}
The syntax error is:
Invalid input '[': expected an identifier character, node labels, a property map, a relationship pattern, '(', '.' or '=' (line 4, column 14)
" SET posts[i].ordinal = i"
^
EDIT/2
Another idea I had was to put the linked-list code inside the FOREACH, but this fails because MATCH cannot be used inside FOREACH. First I created and attached a dummy (b:Blog)-[:FIRST]->(p:Post) relationship, then wrote this query (which doesn't work):
MATCH (f)<-[:FIRST]-(b:Blog)-[:HAS]->(p:Post)
WITH b, collect(p) as posts
FOREACH( x IN posts |
MATCH (b)-[r:FIRST]->(f)
DELETE r
CREATE (b)-[r2:FIRST]->(x), (x)-[:PREV]->(f)
)
RETURN b-[*0..1]-()

Answer 1

As a crutch until you find a better workarund you could try nesting FOREACH (a IN [posts[i]] | FOREACH (b IN [posts[i-1]] | CREATE a-[:PREV]->b)) inside your original foreach loop, as a way to unpack the index slices. For edit 1 try SET (posts[i]).ordinal = i. I think I have a nicer workaround somewhere but can't remember at the moment, if I find it I'll put it up.

Answer 2

Try this Cypher Query
After Edit
MATCH (a:Blog)-[r:HAS]->(p:Post)
WITH a,r,collect(p) as posts,range(1,length(posts)) as rel
MERGE(firstPost:Post {created: head(posts).created , otherprop: head(posts).otherprop})
CREATE a-[:FIRST]->(firstPost)
FOREACH(i IN rel |
MERGE(thisPost:Post {created: (posts[i]).created , otherprop: (posts[i]).otherprop})
MERGE(prevPost:Post {created: (posts[i-1]).created , otherprop: (posts[i-1]).otherprop})
CREATE (prevPost)-[:NEXT]->(thisPost))
WITH a
MATCH a-[r:HAS]->() delete r
This will create something like (Blog)-[FIRST]->(Post)-[NEXT]->(Post)...
By default Cypher sorts the element in COLLECT(p) in natural ascending order. Hence if you have only created property it will sort in the most recent one at last, hence the last node in the next chain starting from BLOG will be your latest POST associated to that BLOG