我很难相信这个问题还没有在某个地方被问到和回答,但我找不到任何线索.
我有一个MongoDB聚合查询,需要按布尔值分组:另一个字段的存在.
例如,让我们从这个系列开始:
> db.test.find()
{ "_id" : ObjectId("53fbede62827b89e4f86c12e"),
"field" : ObjectId("53fbede62827b89e4f86c12d"), "name" : "Erik" }
{ "_id" : ObjectId("53fbee002827b89e4f86c12f"), "name" : "Erik" }
{ "_id" : ObjectId("53fbee092827b89e4f86c131"),
"field" : ObjectId("53fbee092827b89e4f86c130"), "name" : "John" }
{ "_id" : ObjectId("53fbee122827b89e4f86c132"), "name" : "Ben" }
2个文档有"字段",2个没有.
我try 过使用$project,但$exists不存在,而且$cond和$ifNull对我没有帮助.该字段似乎始终存在,即使它不存在:
> db.test.aggregate(
{$project:{fieldExists:{$cond:[{$eq:["$field", null]}, false, true]}}},
{$group:{_id:"$fieldExists", count:{$sum:1}}}
)
{ "_id" : true, "count" : 4 }
我希望以下更简单的聚合可以工作,但由于某些原因,$exists不支持这种方式:
> db.test.aggregate({$group:{_id:{$exists:"$field"}, count:{$sum:1}}})
assert: command failed: {
"errmsg" : "exception: invalid operator '$exists'",
"code" : 15999,
"ok" : 0
} : aggregate failed
Error: command failed: {
"errmsg" : "exception: invalid operator '$exists'",
"code" : 15999,
"ok" : 0
} : aggregate failed
at Error (<anonymous>)
at doassert (src/mongo/shell/assert.js:11:14)
at Function.assert.commandWorked (src/mongo/shell/assert.js:244:5)
at DBCollection.aggregate (src/mongo/shell/collection.js:1149:12)
at (shell):1:9
2014-08-25T19:19:42.344-0700 Error: command failed: {
"errmsg" : "exception: invalid operator '$exists'",
"code" : 15999,
"ok" : 0
} : aggregate failed at src/mongo/shell/assert.js:13
有人知道如何从这样的Collection 中得到想要的结果吗?
预期结果:
{ "_id" : true, "count" : 2 }
{ "_id" : false, "count" : 2 }