With $where you can do something like:

posts.find($where => Code.new("i < 3",

))

and there will be scope created with i variable set to 2 fror "i < 3" part.

However, this doesn't work with map-reduce functions.

map = "function() { emit(this._id,

"
reduce = Code.new(
"function(key, values)

",

)
posts.map_reduce(map, reduce)

This throws an error like:
map-reduce failed: assertion: invoke failed: JS Error: ReferenceError: x is not defined nofile_b:3