Rethinkdb group sequence by N elements - rethinkdb

Question

I have a sequence inside a document
{
errorTicker: [1,2,3,4,5,6,7,8,9]
}
I would like to do a group by N elements and get
for N = 3
{
errorTicker: [[1,2,3],[4,5,6],[7,8,9]]
}
What would have been ideal solution:
r.expr([1,2,3,4,5,6,7,8,9]).group(function(entry, index){
return r.expr(index).div(3).coerceTo('string').split('.')(0);
})
but group does not accept current index, only the entry being processed...
What magic could I use here to achieve the desired result?
P.S. order of the sequence is important to me i.e. group [1,2,3] is not the same as [1,3,5]

Answer 1

I'm almost on the right track, thanks to .range() command
Here is the intermediate version i'm working on right now
r.expr({tickers: [9,6,3,9,6,7,8,1,2]})
.do(function (e){
return r.range(e('tickers').count())
.map(function(index){
return [index, e('tickers')(index)]
})
.group(function(indexValue){
return r.expr(indexValue(0)).div(3).coerceTo('string').split('.')(0);
})
})
The final version that seems to do what I need:
r.expr({tickers: [9,6,3,9,6,7,8,1,2]})
.do(function (e){
return r.range(e('tickers').count())
.map(function(index){
return [index, e('tickers')(index)]
})
.group(function(indexValue){
return r.expr(indexValue(0)).div(3).coerceTo('string').split('.')(0);
})
.ungroup()
.map(function(e){
return e('reduction').map(function(e){
return e(1)
})
})
})
Would appreciate any improvements ;)
updated: to get more efficience we can replace
r.expr(indexValue(0)).div(3).coerceTo('string').split('.')(0)
with
r.expr(index).sub(r.expr(index).mod(3)).div(3);
P.S. rethinkdb totally rocks! it's the most powerful and intuitive NoSQL database I've ever used :)