Problem:

When connecting to a 1.6.x DB, running a mapReduce only needs two params, e.g.

result = db.articles.map_reduce(map, reduce)

However, running the same M/R against 1.7.x you get

pymongo.errors.OperationFailure: command SON([('mapreduce', u'articles'), ('map', Code('function() {\nnew_date = new Date(this.link_time);\nout_date = new_date.getFullYear() + "" + (new_date.getMonth() + 1) + "" + new_date.getDate()\nemit(out_date,

{count:1}

);\n}', {})), ('reduce', Code("function(key, values) {\nvar count = 0;\nvalues.forEach(function(v)

{\n count += v['count'];\n}

);\nreturn

{timeline: key, count : count}

;\n}", {}))]) failed: db assertion failure

Runing this in the shell you see the following on 1.7.x

Mon Feb 14 20:45:58 uncaught exception: map reduce failed: {
"assertion" : "'out' has to be a string or an object",
"assertionCode" : 13606,
"errmsg" : "db assertion failure",
"ok" : 0

Workaround:
Add the extra out parameter, e.g.

result = db.articles.map_reduce(map, reduce, out="i180days")

Solution:
1) Force the output to either be mandatory of default to inline for backward compatibility
2) Update the API and Examples

• http://api.mongodb.org/python/1.9%2B/api/pymongo/collection.html
• http://api.mongodb.org/python/1.9%2B/examples/map_reduce.html
  3) Add a test case for backward compatibility

See Also:
http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-Outputoptions

Business Case:

• backward compatibility is broken with 1.8.x