Arangodb: can I use a subquery to determine starting node for graph traversal - subquery

Question

I am using arangodb to build a permissions system.
I would like to use a subquery to determine starting node for a graph traversal. This is what I attempted
FOR vert IN 1..5
OUTBOUND (
FOR v, e, p IN 1..2
OUTBOUND 'users/jill'
GranterGrant, GrantGrantee
FILTER e.edgeType == 'GrantGrantee'
AND v.type == 'User'
AND v._id == 'users/jim'
LIMIT 1
RETURN p.vertices[1]
)
RolePrivilege, GrantRole, GrantPrivilege
FILTER vert.type == 'Privilege'
AND vert._id == 'privileges/JournalRead'
LIMIT 1
RETURN vert._id
The inner query is meant to find a permissions grant node from jill to jim, and the outer query determines if there is a path from the grant node to the privilege JournalRead
The subquery works when run by itself, and returns a vertex that I would like to use as the starting node for the outer query. The outer query works if I hard code the starting node.
However, when I run the entire query above, arango responds with
Warnings:
[10], 'Invalid input for traversal: Only id strings or objects with _id are allowed'
Result:
[]
Note, I also tried RETURN p.vertices[1]._id within the subquery with the same outcome.
So, is it possible to use a subquery to determine the starting node of a graph traversal?

Answer 1

You can to get the list of starting nodes first and assign them to a variable instead of using a subquery.
So in you case, it would be something like:
let initialNodes = (
FOR v, e, p IN 1..2
OUTBOUND 'users/jill'
GranterGrant, GrantGrantee
FILTER e.edgeType == 'GrantGrantee'
AND v.type == 'User'
AND v._id == 'users/jim'
LIMIT 1
RETURN p.vertices[1]
)
FOR initialNode IN initialNodes
FOR vert IN 1..5
OUTBOUND initialNode
RolePrivilege, GrantRole, GrantPrivilege
FILTER vert.type == 'Privilege'
AND vert._id == 'privileges/JournalRead'
LIMIT 1
RETURN vert._id
Note that you are not limited to a single initial node. You can return multiple one in the subquery if needed.