title: MongoDB MapReduce - Emit one key/one value doesnt call reduce
body: <p>So i'm new with mongodb and mapreduce in general and came across this &quot;quirk&quot; (or atleast in my mind a quirk)</p>
  <p>Say I have objects in my collection like so:</p>
  <blockquote>
  <p>{'key':5, 'value':5}</p>
  <p>{'key':5, 'value':4}</p>
  <p>{'key':5, 'value':1}</p>
  <p>{'key':4, 'value':6}</p>
  <p>{'key':4, 'value':4}</p>
  <p>{'key':3, 'value':0}</p>
  </blockquote>
  <p>My <strong>map</strong> function simply emits the key and the value</p>
  <p>My <strong>reduce</strong> function simply adds the values <strong>AND</strong> before returning them adds 1 (I did this to check to see if the reduce function is even called)</p>
  <p>My results follow:</p>
  <blockquote>
  <p>{'_id': 3, <strong>'value': 0</strong>}</p>
  <p>{'_id':4, 'value': 11.0}</p>
  <p>{'_id':5, 'value': 11.0}</p>
  </blockquote>
  <p>As you can see, for the keys 4 &amp; 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!</p>
  <p>Is this natural behavior of mapreduce in general? For MongoDB? For pymongo (which I am using)?</p>