pymongo User Assertion: 13606:'out' has to be a string or an object

Question

I can't figure out what I'm doing wrong here. I'm using pymongo and have the following map/reduce code (all attributes of the document are directly accessible - i.e. no embedded parts relevant here:

(in file getTableMap.js):

function() {
   var tablePoints1 = 0;
   var tablePoints2 = 0;
   if (this.pointsTeam1 == this.pointsTeam2) {
     tablePoints1 = 1;
     tablePoints2 = 1;
   }
   else {
     if (this.pointsTeam1 > this.pointsTeam2) {
       tablePoints1 = 3;
     }
     else {
       tablePoints2 = 3;
     }
   }
   emit(this.idTeam1, [tablePoints1, this.pointsTeam1, this.pointsTeam2]);
   emit(this.idTeam2, [tablePoints2, this.pointsTeam2, this.pointsTeam1]);
}

The python code which calls the map_reduce looks like this:

def getTableOnMatchday(self):
  m = Code(open('getTableMap.js','r').read())
  r = Code("""function(k,values) { 
    var foo = 'foo';
    return(foo);
  }""")

  result = bl_1.map_reduce(m, r, "myresult")
  for doc in result.find():
    print doc

For the Python code I adapted the simple example straight from the docs: http://api.mongodb.org/python/current/examples/map_reduce.html Map Reduce example from pymongo 2.0.1 documentation

The Python traceback I get when I run the code is:

>>> api.getTableOnMatchday()
  Traceback (most recent call last):
  pymongo.errors.OperationFailure: command SON([('mapreduce', u'bl1_2011'), 
  ...
  ...
  ...
  ) failed: db assertion failure

That didn't exactly tell me very much so I turned the mongod logging on to verbose and found this in the logs:

Thu Sep 15 21:04:02 [conn7] User Assertion: 13606:'out' has to be a string
or an object

From looking at the Python code that actually generates the map_reduce call, the third parameter ('out', according to the pymongo 2.0.1 docs) is 'myresult', which certainly is a string.

What is pymongo complaining about here? The Javascript is syntactically correct (I think). I know the reduce currently does nothing, but this should not prevent the compiling of the command serverside - or maybe it does?

Answer 1

I think I've found the answer, through more trial and error and through reading the documentation for the PHP driver:

result = bl_1.map_reduce(m, r, out="foo")

You actually have to specify out=string as third parameter.

The example in the documentation leads astray here as it says to do:

result = bl_1.map_reduce(m, r, "foo")