MongoDB MapReduce - Emit one key/one value doesnt call reduce

Question

So i'm new with mongodb and mapreduce in general and came across this "quirk" (or atleast in my mind a quirk)

Say I have objects in my collection like so:

{'key':5, 'value':5}

{'key':5, 'value':4}

{'key':5, 'value':1}

{'key':4, 'value':6}

{'key':4, 'value':4}

{'key':3, 'value':0}

My map function simply emits the key and the value

My reduce function simply adds the values AND before returning them adds 1 (I did this to check to see if the reduce function is even called)

My results follow:

{'_id': 3, 'value': 0}

{'_id':4, 'value': 11.0}

{'_id':5, 'value': 11.0}

As you can see, for the keys 4 & 5 I get the expected answer of 11 BUT for the key 3 (with only one entry in the collection with that key) I get the unexpected 0!

Is this natural behavior of mapreduce in general? For MongoDB? For pymongo (which I am using)?

Answer 1

The reduce function combines documents with the same key into one document. If the map function emits a single document for a particular key (as is the case with key 3), the reduce function will not be called.