(function() { 'use strict'; var source = db.mr_source; var out = db.mr_out; source.drop(); out.drop(); // create a dummy collection for mapReduce input source.insert({a: 1}); // create a collection with no _id index and insert a document db.createCollection(out.getName(), {autoIndexId: false}); out.insert({_id: 0}); var mapper = function() {emit(1, 1);}; var reducer = function() {return 1;}; // the mapReduce will fail because of the missing _id index source.mapReduce(mapper, reducer, {out: {reduce: out.getName()}}); }());