MongoDB Map/Reduce raise exception: failed: db assertion failure

Question

I created map/reduce functions to group tasks results in one result object. I wrote in in python with using pymongo library:

    m = Code("""function() {
        data = {};
        res = ''
        if(this.result_id) {
            res={'objectid':this.result_id.toString()};
        } else {
            res=this.result;
        }
        emit(this.data, res);
    }""")
    r = Code("""function(k,values) { 
        data={};
        for(var i=0; i<values.length; i++ ) {
            for(attr in values[i])
                data[attr]=values[i][attr];
        }
        return data
    }""")

And i need that result object be in the same order as input tasks queries. But when i use sort param in request:

   res = db.tasks.map_reduce(m, r, query={'job_id':job_id},sort={'position':pymongo.ASCENDING})

But this raise exception in mongodb:

    Traceback (most recent call last):
      File "/usr/local/lib/python2.6/dist-packages/gevent/greenlet.py", line 403, in run
        result = self._run(*self.args, **self.kwargs)
      File "/data/www/public/app/seotools/daemon/scripts/mainconverter.py", line 129, in work
        res = autoreconnect(self.db.tasks.map_reduce,m, r, query={'job_id':job_id},sort={'position':1})
      File "/data/www/public/app/seotools/daemon/lib/db/mongo.py", line 95, in autoreconnect
        result = func(*args,**kwargs)
      File "/usr/local/lib/python2.6/dist-packages/pymongo-1.8.1-py2.6-linux-x86_64.egg/pymongo/collection.py", line 945, in map_reduce
        map=map, reduce=reduce, **kwargs)
      File "/usr/local/lib/python2.6/dist-packages/pymongo-1.8.1-py2.6-linux-x86_64.egg/pymongo/database.py", line 294, in command
        (command, result["errmsg"]))
    OperationFailure: command SON([('mapreduce', u'tasks'), ('sort', {'position': 1}), ('query', {'job_id': ObjectId('4d0b30909c7684b60e000000')}), ('reduce', Code('function(k,values) { 
        data={};
        for(var i=0; i<values.length; i++ ) {
            for(attr in values[i])
                data[attr]=values[i][attr];
       }
        return data
    }', {})), ('map', Code("function() {
        data = {};
        res = ''
        if(this.result_id) {
            res={'objectid':this.result_id.toString()};
        } else {
            res=this.result;
        }
        emit(this.data, res);
    }", {}))]) failed: db assertion failure

When i use the same query without sort param:

   res = db.tasks.map_reduce(m, r, query={'job_id':job_id})

It's work pretty fine.

Where can be the problem?

Answer 1

I found my problem. As was said by one of developers, we can't use sorting without indexes. So if you use sorting, first of all you must create an index for that sorting.