Adding New Relationships in Neo4j Database using apoc.periodic.iterate

I have a Neo4j database with two kinds of nodes - Authors and Articles. Some of the articles have more than one author. I am trying to create an undirected relationship between the authors who worked together on an article. My current non-functional query is this:

CALL apoc.periodic.iterate(
    "MATCH (a:Author) WHERE (a)-[:WROTE]->()<-[:WROTE]-(b:Author) RETURN a,b",
    "WITH {a} AS a, {b} as b CREATE (a)-[r:COAUTHOR]-(b)", {batchSize:10000, parallel:true})

I get the following error:

Failed to invoke procedure apoc.periodic.iterate: Caused by: org.neo4j.exceptions.SyntaxException: PatternExpressions are not allowed to introduce new variables: 'b'. (line 1, column 60 (offset: 59)) "EXPLAIN MATCH (a:Author) WHERE (a)-[:WROTE]->()<-[:WROTE]-(b:Author) RETURN a,b"

I can see that the issue is that I am trying to do too much in the first MATCH statemetn, but I'm new to Cypher and am having trouble breaking it up.

Thanks very much, John

Solution

The problem stems from your first statement:

MATCH (a:Author)
WHERE (a)-[:WROTE]->()<-[:WROTE]-(b:Author)
RETURN a,b

Cypher does not allow introducing new variables on the WHERE part of the query, so your (b:Author) will not be allowed since it's located on the WHERE clause.

To fix, move (b:Author) to the MATCH statement. Also, if you are finding a pattern on the WHERE clause, you should use the exists() function in order to return a boolean.

MATCH (a:Author), (b:Author)
WHERE exists((a)-[:WROTE]->()<-[:WROTE]-(b))
RETURN a, b