MongoDB:_id不能是一个数组

我有一个大型数据集(大约1.1M文档),我需要运行mapreduce.

要分组的字段是名为xref的数组.由于集合的大小以及我在32位环境中这样做的事实,我正在尝试将集合减少到新数据库中的另一个集合.

首先,这是一个数据样本:

{ "_id" : ObjectId("4ec6d3aa61910ad451f12e01"), 
"bii" : -32.9867, 
"class" : 2456, 
"decdeg" : -82.4856, 
"lii" : 297.4896,
"name" : "HD 22237", 
"radeg" : 50.3284,
"vmag" : 8, 
"xref" : ["HD 22237", "CPD -82   65", "-82   64","PPM 376283", "SAO 258336",
          "CP-82   65","GC 4125" ] }

{ "_id" : ObjectId("4ec6d44661910ad451f78eba"), 
  "bii" : -32.9901, 
  "class" : 2450, 
  "decdeg" : -82.4781, 
  "decpm" : 0.013,
  "lii" : 297.4807, 
  "name" : "PPM 376283", 
  "radeg" : 50.3543, 
  "rapm" : 0.0357, 
  "vmag" : 8.4, 
  "xref" : ["HD 22237", "CPD -82   65", "-82   64","PPM 376283", "SAO 258336",
          "CP-82   65","GC 4125" ] }

{ "_id" : ObjectId("4ec6d48a61910ad451feae04"), 
  "bii" : -32.9903, 
  "class" : 2450, 
  "decdeg" : -82.4779, 
  "decpm" : 0.027,
  "hd_component" : 0, 
  "lii" : 297.4806, 
  "name" : "SAO 258336", 
  "radeg" : 50.3543, 
  "rapm" : 0.0355, 
  "vmag" : 8, 
"xref" : ["HD 22237", "CPD -82   65", "-82   64","PPM 376283", "SAO 258336",
          "CP-82   65","GC 4125" ] }

这是map和reduce函数(现在我只是lii和bii字段):

function map() {
try {
    emit(this.xref, {lii:this.lii, bii:this.bii});
} catch(e) {
}
}

function reduce(key, values) {

var result = {xref:key, lii: 0.0, bii: 0.0};
try {
    values.forEach(function(value) {

        if (value.lii && value.bii) {
            result.lii += value.lii;
            result.bii += value.bii;
        }
    });

    result.bii /= values.length;
    result.lii /= values.length;
} catch(e) {
}

return result;
}

不幸的是,运行它最终会出现一条错误消息:

db.catalog.mapReduce(map, reduce, {out:{replace:"catalog2", db:"astro2"}});

Wed Nov 23 10:12:25 uncaught exception: map reduce failed:{
    "assertion" : "_id cannot be an array",
    "assertionCode" : 10099,
    "errmsg" : "db assertion failure",
    "ok" : 0

外部参照字段是一个数组,但该数组中的所有值都相等.它是否尝试将该数组用作新集合中的id字段？

是的,无法将_id设置为数组,因为它具有索引的特殊行为.您发出的键在输出集合中用作_id.如果结果很小,这可能只适用于"内联"输出模式,因为它不会进入集合.但理想情况下,您可以将数组转换为字符串(例如,将值连接起来)并将其用作_id,或者将其设置为子对象而不是数组.

另请注意,reduce函数的结果不应包含键.回来{lii:..,bii:..}