Map/Reduce to extract last seen value

I have timestamped documents which contain an array of instances:

Example document:

	"type": "occurance",
	"present": [
		{"id": ["x", "y"]},
		{"id": ["x", "b"]},
		{"id": ["y", "b"]}
	"t": "2017-03-24T12:01:10.560Z"

Map is a classic one:

doc.present.forEach(function(u_id) {emit(, doc.t);})

which returns, normally, all keys, values.

However, when I reduce I get a null key and a really large value.
I group keys, values or combine values (since rereduce is a thing).
Data seem/are correct

Is there an option however, that I could get the results in the same fashion as the map query, only reduced?

This is my first map/reduce, so apologies for breaking all conventions on handling them properly