Mongodb子文檔字段聯合

Question

我需要加入mongodb的兩個集合。 首先，我有一個像這樣的聚合：

db.messages.aggregate([
{
 $lookup: {
        from :"channels",
        localField: "channels",
        foreignField: "_id",
        as: "merged_channels"
 }
}
]).pretty()

整合之后，我的文檔如下所示：

{
    "_id" : "21ca6117-1f14-4613-9407-db7f3a011142",
    "author" : "03072fad-a8fd-53f3-b25f-abbfaf15b055",
    "title" : "test2",
    "body" : "test2",
    "channels" : [
        "8008d5a8-eb3b-4e55-98c5-a60fd7275bd2",
        "8008d5a8-eb3b-4e55-98c5-a60fd7275bd3"
    ],
    "comments" : [
        {
            "author" : "03072fad-a8fd-53f3-b25f-abbfaf15b055",
            "body" : "comment1",
            "created_at" : ISODate("2018-03-15T07:08:10.018Z")
        },
        {
            "author" : "03072fad-a8fd-53f3-b25f-abbfaf15b055",
            "body" : "testbody",
            "created_at" : ISODate("2018-03-15T07:08:09.366Z")
        }
    ],
    "created_at" : ISODate("2018-03-15T07:08:09.018Z"),
    "updated_at" : ISODate("2018-03-15T07:08:09.366Z"),
    "deleted_at" : null,
    "merged_channels" : [
        {
            "_id" : "8008d5a8-eb3b-4e55-98c5-a60fd7275bd2",
            "author" : "03072fad-a8fd-53f3-b25f-abbfaf15b055",
            "name" : "chan2",
            "members" : [
                {
                    "id" : "8008d5a8-eb3b-4e55-98c5-a60fd7275cb5",
                    "type" : "group"
                }
            ],
            "created_at" : ISODate("2018-03-15T07:08:08.872Z"),
            "updated_at" : ISODate("2018-03-15T07:08:08.872Z"),
            "deleted_at" : null
        },
        {
            "_id" : "8008d5a8-eb3b-4e55-98c5-a60fd7275bd3",
            "author" : "8008d5a8-eb3b-4e55-98c5-a60fd7275fg4",
            "name" : "chan3",
            "members" : [
                {
                    "id" : "8008d5a8-eb3b-4e55-98c5-a60fd7275cb5",
                    "type" : "group"
                },
                {
                    "id" : "8008d5a8-eb3b-4e55-98c5-a60fd7275cb6",
                    "type" : "user"
                },
                {
                    "id" : "8008d5a8-eb3b-4e55-98c5-a60fd7275cb7",
                    "type" : "role"
                }
            ],
            "created_at" : ISODate("2018-03-15T07:08:08.872Z"),
            "updated_at" : ISODate("2018-03-15T07:08:09.358Z"),
            "deleted_at" : null
        }
    ]
}

我想從chan2和chan3中獲取成員字段，將它們合並並放入根（消息）文檔中。 然后刪除merged_channels字段。 刪除合並的字段不是問題，但不知道如何從子對象中提取字段並合並它們。

我將如何實現？

Answer 1

您可以在3.4中使用以下匯總。

$reduce到$concatArrays並投影，排除掉merged_channels字段。

db.col.aggregate({
  "$addFields":{
    "merged_arrays":{
      "$reduce":{
        "input":"$merged_channels",
        "initialValue":[],
        "in":{"$concatArrays":["$$value", "$$this.members"]}
      }
    }
  },
  {"$project":{"merged_channels":0}}
})