连接MongoDB中不同数据库的两个collecions

我想加入以下两个学院。 但是，这些集合是不同的数据库

> use test
> db.payment_history.find()
{ "_id" : ObjectId("5752bbbaf12f5908ec24b3a9"), "user_id" : 1233, "cash_id" : 3 }
{ "_id" : ObjectId("5752bbbcf12f5908ec24b3aa"), "user_id" : 1233, "cash_id" : 1 }
{ "_id" : ObjectId("5752bba5f12f5908ec24b3a5"), "user_id" : 1234, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbaaf12f5908ec24b3a6"), "user_id" : 1235, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbaff12f5908ec24b3a7"), "user_id" : 1235, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbb5f12f5908ec24b3a8"), "user_id" : 1236, "cash_id" : 3 }
> use test_mst
> db.mst_cash.find()
{ "_id" : ObjectId("5752bbebf12f5908ec24b3ab"), "cash_id" : 1, "num" : 100 }
{ "_id" : ObjectId("5752bbf1f12f5908ec24b3ac"), "cash_id" : 2, "num" : 200 }
{ "_id" : ObjectId("5752bbf8f12f5908ec24b3ad"), "cash_id" : 3, "num" : 500 }
{ "_id" : ObjectId("5752bbfff12f5908ec24b3ae"), "cash_id" : 4, "num" : 1000 }

我想输出如下所示的连接。 （关系键为cash\u id）

我尝试像下面这样编写javascript

> var map_1 = function(){ emit(this._id, {user_id: this.user_id, cash_id: this.cash_id}) };

> var map_2 = function(){ emit(this.cash_id, {num: this.num}) };

> var r = function(key, values){
    var result = {"cash_id": 0};
    values.forEach(function(v){
        result.cash_id = v.cash_id;
    });

    return result;
  };

> var r2 = function(key, values){
      var result = {"num": 0};
      values.forEach(function(v){
          result.num = v.num;
       });

      return result;
  };

> db.payment_history.mapReduce(map_1, r, {out: {reduce: 'joined'}});
> db.getSiblingDB('test_mst').mst_cash.mapReduce(map_2, r2, {out: {reduce: 'joined'}});

但是，加入collecion并不是mst_现金数据的合并结果。 我确保'db.getSiblingDB（'test_mst'）.mst_cash.mapReduce（map_2，r2，{out:{reduce:'joined'}}）；'在测试mst数据库中创建加入的集合。 我想将mst_现金数据合并到测试数据库中加入的collecion。

---

我该怎么办？

第一件事-mongo不提供对不同数据库的操作

> use test
> db.payment_history.find()
{ "_id" : ObjectId("5752bbbaf12f5908ec24b3a9"), "user_id" : 1233, "cash_id" : 3 }
{ "_id" : ObjectId("5752bbbcf12f5908ec24b3aa"), "user_id" : 1233, "cash_id" : 1 }
{ "_id" : ObjectId("5752bba5f12f5908ec24b3a5"), "user_id" : 1234, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbaaf12f5908ec24b3a6"), "user_id" : 1235, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbaff12f5908ec24b3a7"), "user_id" : 1235, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbb5f12f5908ec24b3a8"), "user_id" : 1236, "cash_id" : 3 }
> use test_mst
> db.mst_cash.find()
{ "_id" : ObjectId("5752bbebf12f5908ec24b3ab"), "cash_id" : 1, "num" : 100 }
{ "_id" : ObjectId("5752bbf1f12f5908ec24b3ac"), "cash_id" : 2, "num" : 200 }
{ "_id" : ObjectId("5752bbf8f12f5908ec24b3ad"), "cash_id" : 3, "num" : 500 }
{ "_id" : ObjectId("5752bbfff12f5908ec24b3ae"), "cash_id" : 4, "num" : 1000 }

若要加入集合，它们必须是同一个数据库，然后我们可以利用聚合框架，如下面的代码片段：

var lookUp = {
    $lookup : {
        from : "mst_cash",
        localField : "cash_id",
        foreignField : "cash_id",
        as : "operations"
    }
}

var unwind = {
    $unwind : "$operations"
}

var project = {
    $project : {
        _id : 1,
        "user_id" : 1,
        "cash_id" : 1,
        "num" : "$operations.num"
    }
}



db.payment_history.aggregate([lookUp, unwind, project])

该查询的结果是：

{
    "_id" : ObjectId("5752bbbaf12f5908ec24b3a9"),
    "user_id" : 1233.0,
    "cash_id" : 3.0,
    "num" : 500.0
}

---

首先，mongo不提供对不同数据库的操作

> use test
> db.payment_history.find()
{ "_id" : ObjectId("5752bbbaf12f5908ec24b3a9"), "user_id" : 1233, "cash_id" : 3 }
{ "_id" : ObjectId("5752bbbcf12f5908ec24b3aa"), "user_id" : 1233, "cash_id" : 1 }
{ "_id" : ObjectId("5752bba5f12f5908ec24b3a5"), "user_id" : 1234, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbaaf12f5908ec24b3a6"), "user_id" : 1235, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbaff12f5908ec24b3a7"), "user_id" : 1235, "cash_id" : 1 }
{ "_id" : ObjectId("5752bbb5f12f5908ec24b3a8"), "user_id" : 1236, "cash_id" : 3 }
> use test_mst
> db.mst_cash.find()
{ "_id" : ObjectId("5752bbebf12f5908ec24b3ab"), "cash_id" : 1, "num" : 100 }
{ "_id" : ObjectId("5752bbf1f12f5908ec24b3ac"), "cash_id" : 2, "num" : 200 }
{ "_id" : ObjectId("5752bbf8f12f5908ec24b3ad"), "cash_id" : 3, "num" : 500 }
{ "_id" : ObjectId("5752bbfff12f5908ec24b3ae"), "cash_id" : 4, "num" : 1000 }

若要加入集合，它们必须是同一个数据库，然后我们可以利用聚合框架，如下面的代码片段：

var lookUp = {
    $lookup : {
        from : "mst_cash",
        localField : "cash_id",
        foreignField : "cash_id",
        as : "operations"
    }
}

var unwind = {
    $unwind : "$operations"
}

var project = {
    $project : {
        _id : 1,
        "user_id" : 1,
        "cash_id" : 1,
        "num" : "$operations.num"
    }
}



db.payment_history.aggregate([lookUp, unwind, project])

该查询的结果是：

{
    "_id" : ObjectId("5752bbbaf12f5908ec24b3a9"),
    "user_id" : 1233.0,
    "cash_id" : 3.0,
    "num" : 500.0
}

---

我明白。谢谢你的回答，我明白。谢谢你的回答