update other collection when mapReduce

[ruby]

is mapReduce readonly?

try to update other collection when mapReduce :

db.things.remove()
db.things2.remove()

db.things.save({test:1})
db.things2.save({test2:2})

m=function(){
	t=db.things.findOne()
	t.test = this.test2
	db.things.save(t)
	emit(this.test2,1)
}
r=function(k,vals){ return vals[0]}
db.things.find()
db.things2.find()

res=db.things2.mapReduce(m,r)
db.things.find()


but failed.