"value is not a record or a record-id" when passing batch variable to "set =" in update

Question

EDIT : see at bottom for reproduce the problem on a smaller stat/schema set

I'm experimenting around OrientDB time-series use case and I'm trying to chain each levels of two trees. So I did this sql batch :

let $w1 = select expand(W1[2406]) from #12:2;
let $w2 = select expand(W1[2407]) from #12:2;
let $d1 = select expand(D1[4]) from $w1;
let $d2 = select expand(D1[0]) from $w2;
let $h1 = select expand(H1[23]) from $d1;
let $h2 = select expand(H1[0]) from $d2;
let $m1 = select expand(M1[59]) from $h1;
let $m2 = select expand(M1[0]) from $h2;
update $w1 set next = $w2;
update $w2 set previous = $w1;
update $d1 set next = $d2;
update $d2 set previous = $d1;
update $h1 set next = $h2;
update $h2 set previous = $h1;
update $m1 set next = $m2;
update $m2 set previous = $m1;

But I get this error on the first update:

The field 'W1.next' has been declared as LINK but the value is not a record or a record-id

...That I don't understand because when I try :

return [$w1, $w2, $d1, $d2, $h1, $h2, $m1, $m2];

I get the records as expected...

So two questions :

1. Why this error ?
2. Is there a better way to do this since I'm still a beginner ?

(Note : I would like to stay in sql/batch sql)

the classes are like this :

Symbol{
   W1 : LINKMAP W1
}

W1 {
   next : LINK W1
   previous : LINK W1
   D1 : LINKMAP D1
}
D1 {
   next : LINK D2
   previous : LINK D2
   H1 : LINKMAP H1
}
H1 [...]
M1 [...]

EDIT : How to reproduce the problem on a smaller data set :

Schema creation :

create class A extends V;
create class B extends V;
create property A.B LINKMAP B;
create property B.B LINK B;

note : A.B is an array-like of B elements, B.B is a 1to1 link

dummy values to insert :

insert into B CONTENT {}
insert into B CONTENT {}

now, get the rids of the two dummies to insert into A

select from B

now insert as follow the two items (#13:0 and #13:1 in my case)

insert into A(B) values ({'0' : #13:0, '1' : #13:1})
insert into A(B) values ({'0' : #13:0, '1' : #13:1})

Finally, try this batch :

let $b1 = select expand(B[0]) from A;
let $b2 = select expand(B[1]) from A;
update $b1 SET B = $b2;
update $b2 SET B = $b1;

you see the error

The field 'B.B' has been declared as LINK but the value is not a record or a record-id

but if you do that :

let $b1 = select expand(B[0]) from A;
let $b2 = select expand(B[1]) from A;
return $b1;

and

let $b1 = select expand(B[0]) from A;
let $b2 = select expand(B[1]) from A;
return $b2;

and

let $b1 = select expand(B[0]) from A;
let $b2 = select expand(B[1]) from A;
return {'b1': $b1, 'b2' : $b2};

you can see that they are not collections, but really records

EDIT2 The solution provided by Isavio works, but I would like to know why it does since in the previous results, they doesn't seem to be collection?

let $b1 = select expand(B[0]) from A;
let $b2 = select expand(B[1]) from A;
update $b1 SET B = $b2[0];
update $b2 SET B = $b1[0];

Answer 1

Could be possible that the result of update is a collection of records? Can you try using:

.....
update $w1 set next = $w2[0];
....

EDIT

I think using the LET will result always a collection.