mongodb 聚合中的 $replaceRoot

我有一个这样的集合：

{
    "_id" : ObjectId("5bd1686ba64b9206349284db"),
    "type" : "Package",
    "codeInstances" : [ 
        {
            "name" : "a",
            "description" : "a"          
        }, 
        {
            "name" : "b",
            "description" : "b1"
        }, 
        {
            "name" : "b",
            "description" : "b2"
        }
    ]
}
{
    "_id" : ObjectId("5bd16ab8a64b92068d485054"),
    "type" : "Package",
    "codeInstances" : [ 
        {
            "name" : "a",
            "description" : "a"          
        }, 
        {
            "name" : "b",
            "description" : "b3"
        }
    ]
}

以下结构是我想要的：

{
      "name" : "b",
      "description" : "b1"
}, 
{
      "name" : "b",
      "description" : "b1"
}, 
{
      "name" : "b",
      "description" : "b3"
}

我尝试了这个聚合操作：

db.getCollection('t_system_code').aggregate([
  {"$unwind":"$codeInstances"},
  {$match:{"codeInstances.name":"b","type" : "Package"}},
  {"$project":{"codeInstances":1,"_id":0}}
]);

但这未定义不是我想要的结构：

{
    "codeInstances" : {
        "name" : "b",
        "description" : "b1"
    }
}
{
    "codeInstances" : {
        "name" : "b",
        "description" : "b2"
    }
}
{
    "codeInstances" : {
        "name" : "b",
        "description" : "b3"
    }
}

帮助。谢谢。

您可以尝试使用以下聚合$replaceRoot

db.collection.aggregate([
  { "$match": { "codeInstances.name": "b", "type": "Package" }},
  { "$unwind": "$codeInstances" },
  { "$match": { "codeInstances.name": "b", "type": "Package" }},
  { "$replaceRoot": { "newRoot": "$codeInstances" }}
])