failure trying to shorten CASE-based predicate - neo4j

Question

This was inspired by these two SO threads:
Boolean value return from Neo4j cypher query without CASE
How to set property name and value on csv load?
I have a CSV with 3 columns:
first_name,prop_name,prop_value
John,weight,100
Paul,height,200
John,hair_color,blonde
Paul,weight,120
So, there are a number of people, and their properties are randomly scattered across different rows. My goal is to screen the rows and assign all found properties to their holders. For the sake of simplicity, let's focus on the 'weight' property only.
I do know how to make this work the long way:
LOAD CSV WITH HEADERS FROM
"file:///test.csv" AS line
WITH line
MERGE (m:Person {name:line.first_name})
WITH line, CASE line.prop_name WHEN "weight" THEN [1] ELSE [] END as loopW
UNWIND loopW as w
MATCH (p:Person {name: line.first_name})
SET p.weight = line.prop_value
But then, I tried to replace the CASE line with a shorter version
WITH line, collect(line.prop_name = "weight") as loopW
...which resulted in weird behavior, where the created nodes did get their 'weight' keys assigned to, but sometimes with the wrong values. So, I could see something like (:Person {weight:blue})
What would be the right way to get rid of the CASE?

Answer 1

You should know that your current usage filters out all lines that don't have "weight" as the prop_name (the UNWIND of an empty collection wipes out all the other lines and they won't get processed).
What you really need is a better way to set dynamically named properties on your nodes so you can avoid the CASE usage completely.
If you can install APOC Procedures (please read the instructions at the top for how to modify your neo4j.conf to whitelist the procedures, and pay attention to the version matrix to ensure you get the version that corresponds with your Neo4j version) there is one that is a perfect fit for what you're trying to do: CALL apoc.create.setProperty( [node,id,ids,nodes], key, value) YIELD node
Usage would be something like:
LOAD CSV WITH HEADERS FROM
"file:///test.csv" AS line
MERGE (m:Person {name:line.first_name})
CALL apoc.create.setProperty(m, line.prop_name, line.prop_value) YIELD node
RETURN count(distinct m)
EDIT
Expanding on what's wrong with the original query:
UNWIND produces rows in a multiplicative way, with respect to the number of elements in the collection. If the collection in the row had 5 elements, the single row would result in 5 rows, one for each element. If the collection was empty, the row would be removed instead, as there wouldn't be any elements in the collection for which to output rows. Because of this, any further WITH line, CASE ... lines in the query wouldn't do any good.
Let's analyze the original query, going by the example input csv:
LOAD CSV WITH HEADERS FROM
"file:///test.csv" AS line
WITH line //redundant, line not needed
MERGE (m:Person {name:line.first_name}) // 4 rows corresponding with 2 nodes
WITH line, CASE line.prop_name WHEN "weight" THEN [1] ELSE [] END as loopW
// still 4 rows, 2 have [1] as loopW, the other 2 have [] as loopW
UNWIND loopW as w // 2 rows eliminated by unwinding empty collections
MATCH (p:Person {name: line.first_name})
SET p.weight = line.prop_value
// only 2 rows are for 'John,weight,100' and 'Paul,weight,120'
// any further repetitions of WITH line, CASE ... UNWIND for different props will fail and eliminate the remaining 2 rows.