Neo4j Cypher query and index of element in the collection - neo4j

Question

I'm trying to find index number of Decision by {decisionGroupId}, {decisionId} and {criteriaIds}
This is my current Cypher query:
MATCH (dg:DecisionGroup)-[:CONTAINS]->(childD:Decision)
WHERE dg.id = {decisionGroupId}
OPTIONAL MATCH (childD)-[vg:HAS_VOTE_ON]->(c:Criterion)
WHERE c.id IN {criteriaIds}
WITH childD, vg.avgVotesWeight as weight, vg.totalVotes as totalVotes
ORDER BY weight DESC, totalVotes DESC
WITH COLLECT(childD) AS ps
RETURN REDUCE(ix = -1, i IN RANGE(0, SIZE(ps)-1)
| CASE ps[i].id WHEN {decisionId} THEN i ELSE ix END) AS ix
I have only 3 Decision in the database but this query returns the following indices:
2
3
4
while I expecting something like(starting from 0 and -1 if not found)
0
1
2
What is wrong with my query and how to fix it?
UPDATED
This query is working fine with COLLECT(DISTINCT childD) AS ps:
MATCH (dg:DecisionGroup)-[:CONTAINS]->(childD:Decision)
WHERE dg.id = {decisionGroupId}
OPTIONAL MATCH (childD)-[vg:HAS_VOTE_ON]->(c:Criterion)
WHERE c.id IN {criteriaIds}
WITH childD, vg.avgVotesWeight as weight, vg.totalVotes as totalVotes
ORDER BY weight DESC, totalVotes DESC
WITH COLLECT(DISTINCT childD) AS ps
RETURN REDUCE(ix = -1, i IN RANGE(0, SIZE(ps)-1)
| CASE ps[i].id WHEN {decisionId} THEN i ELSE ix END) AS ix
Please help me to refactor this query and get rid of heavy REDUCE.

Answer 1

Let's try to get the reduce part right with a simpler query:
WITH ['a', 'b', 'c'] AS ps
RETURN
reduce(ix = -1, i IN RANGE(0, SIZE(ps)-1) |
CASE ps[i] WHEN 'b' THEN i ELSE ix END) AS ix
)
As I stated in the comments, it is usually better to avoid reduce if possible. So, to express the same using a list comprehension, use WHERE for filtering.
WITH ['a', 'b', 'c'] AS ps
RETURN [i IN RANGE(0, SIZE(ps)-1) WHERE ps[i] = 'b'][0]
The list comprehension results in a list with a single element, and we will use the [0] indexer to select that element.
After adapting this to your query, we'll get something like this:
MATCH (dg:DecisionGroup)-[:CONTAINS]->(childD:Decision)
WHERE dg.id = {decisionGroupId}
OPTIONAL MATCH (childD)-[vg:HAS_VOTE_ON]->(c:Criterion)
WHERE c.id IN {criteriaIds}
WITH childD, vg.avgVotesWeight as weight, vg.totalVotes as totalVotes
ORDER BY weight DESC, totalVotes DESC
WITH COLLECT(DISTINCT childD) AS ps
RETURN [i IN RANGE(0, SIZE(ps)-1) WHERE ps[i].id = {decisionId}][0]

Answer 2

If you have APOC installed, you can also use the function:
return apoc.coll.indexOf([1,2,3],2)