MongoDB Map/Reduce 报错：失败，数据库断言失败

Question

我写了一个map/reduce的功能，用来把任务的结果整合到一个结果对象里。我是用Python和pymongo库来实现的：

    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
    }""")

我希望这个结果对象的顺序和输入的任务查询顺序是一样的。但是当我在请求中使用排序参数时：

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

这在mongodb中会引发异常：

    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

但当我使用没有排序参数的相同查询时：

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

这个就能正常工作。

那问题可能出在哪里呢？

Answer 1

我找到了我的问题。正如一位开发者所说的，我们不能在没有索引的情况下进行排序。所以如果你想要排序，首先必须为这个排序创建一个索引。