The check to determine if the joined document is the correct type should be performed after _apply

[phillips1012]

If I want to do something like

.getJoin
  comments:
    _apply: (x) -> x.count()

It shouldn't throw this

Cannot coerce NUMBER to ARRAY

It'd be nice to see a fix to this, as I'd rather not load 300 objects into memory 10 times every single request.

[neumino]

Hello @phillips1012

I just released 1.16.2 with a fix for that.
You have to pass an extra option _array: false if you do not want to coerce to an array.

Something like this in your case:

Posts.getJoin
    comments:
        _apply: (x) -> x.count()
        _array: false

The reason why thinky doesn't check the type itself is because you currently cannot bind a stream with do, and cannot do something like
r.branch(seq.typeOf().eq("STREAM"), seq.coerceTo("ARRAY"), seq) without having to compute twice seq (which can be expensive).

If streams are automatically coerce to arrays, this would remove the need for _array but this is not yet implemented - rethinkdb/rethinkdb#2721

Thanks for the suggestion!