Subquery for each item in a Couchbase double nested array - couchbase

Question

Having the next objects
"a" : {
"id" : "1",
"arr" : [
{"id" : "b1"}, {"id" : "b2"}
]
}
"b1" : {
"id" : "b1",
"innerArr" : [{"id" : "c1"},{"id" : "c2"}]
}
"b2" : {
"id" : "b2",
"innerArr" : [{"id" : "c3"}]
}
"c1" : {
"name" : "c1"
}...
Right now I'm able to make a join with NEST over an array like this.
SELECT *
FROM bucket AS a
NEST bucket AS bs
ON META(a).id IN a.arr[*].id
{
"id" : "1",
"arr" : [
{"id" : "b1"}, {"id" : "b2"}
],
"bs" : [
{
"id" : "b1",
"innerArr" : [{"id" : "c1"},{"id" : "c2"}]
},
{
"id" : "b2",
"innerArr" : [{"id" : "c3"}]
}
]
}
Now I want to NEST c documents for each item in bs
Adding this NEST doesn't work
NEST bucket AS c
ON META(c).id IN bs[*].innerArr[*].id
I'm looking for this result:
{
"id" : "1",
"arr" : [
{"id" : "b1"}, {"id" : "b2"}
],
"bs" : [
{
"id" : "b1",
"innerArr" : [{"id" : "c1"},{"id" : "c2"}],
"cs" : [{"name" : "c1"},{"name" : "c2"}]
},
{
"id" : "b2",
"innerArr" : [{"id" : "c3"}],
"cs" : [{"name" : "c3"}]
}
]
}

Answer 1

I was able to solve it iterating every element from bs in a subquery. Since the subquery is in the SELECT part of the query, it must have USE KEYS instead of ON META().id =. Finally I add the subquery result to each item.
SELECT a*.
,ARRAY OBJECT_ADD(item, "cs", (SELECT c.* FROM bucket AS c USE KEYS item.innerArr[*].id)) FOR item IN bs END
FROM bucket AS a,
NEST bucket AS bs
ON META(a).id IN a.arr[*].id
It means that for each element in bs it queries every element of innerArray. And then adds the result to the element of bs.

Answer 2

NEST bucket AS bs .
bs starts with document (Scan, Fetch, ON). At the end of NEST bs becomes ARRAY for Filter, Group, projections, etc. Same applies chained JOIN, NEST. Example 17: https://blog.couchbase.com/ansi-join-support-n1ql/
In those situations use JOIN+GROUP on LEFT document + ARRAG_AGG on right document. Or use ARRAY …FOR… syntax.
The desired results can be achieved by following query. This LEFT outer nest
SELECT a.*,
(SELECT b.*,
(SELECT c.*
FROM bucket AS c USE KEYS b.innerArr[*].id) AS cs
FROM bucket AS b USE KEYS a.arr[*].id) AS bs
FROM bucket AS a
WHERE ..........;