Neo4j, empty list in first unwind stop second unwind from executing - neo4j

Question

I have two unwinds, that create some relationships and nodes, but if the list for the first unwinds is empty, the second unwind doesn't execute.
How can I fix that?
CALL apoc.periodic.iterate(
"
UNWIND $POSTS as post
RETURN post
",
"
MERGE (p:Post{id: post.id})
WITH p, post
UNWIND post.tags as value
MERGE (t:Tag{tag: value})
MERGE (t)-[:has_tag]->(p)
WITH p, post
UNWIND post.user_mentions as user_mention
MERGE (u1:User{id: user_mention})
MERGE (p)-[:mentions]->(u1)
",
{batchSize: 500, params: {POSTS: $POSTS}, iterateList:true}
)
Example results
Parameters, with non-empty tags
[
{
"id": 123,
"tags": [1],
"user_mentions": [123, 234],
}
]
Graph created in database - Expected result
Parameters, with empty tags
[
{
"id": 123,
"tags": [],
"user_mentions": [123, 234],
}
]
Graph created in the database (Lacking 'mentions' relationships) - Unexpected result

Answer 1

This is expected, UNWIND produces rows and thus if the list is empty no rows are produced and the query doesn't continue.
You need to switch to FOREACH for this use case :
CALL apoc.periodic.iterate(
"
UNWIND $POSTS as post
RETURN post
",
"
MERGE (p:Post{id: post.id})
WITH p, post
FOREACH(value IN post.tags |
MERGE (t:Tag{tag: value})
MERGE (t)-[:has_tag]->(p)
)
WITH p, post
FOREACH(user_mention IN post.user_mentions |
MERGE (u1:User{id: user_mention})
MERGE (p)-[:mentions]->(u1)
)
",
{batchSize: 500, params: {POSTS: $POSTS}, iterateList:true}
)