你必须按照你描述的方法去做。MongoDB是非关系数据库，不支持连接。

下面是一个“join”* Actors和Movies集合的例子:

https://github.com/mongodb/cookbook/blob/master/content/patterns/pivot.txt

它使用了.mapReduce()方法

join -在面向文档的数据库中加入的替代方案

MongoDB不允许连接，但是你可以使用插件来处理。检查mongo-join插件。这是最好的，我已经用过了。你可以直接使用npm安装它，就像这个npm install mongo-join。您可以通过示例查看完整的文档。

(++)非常有用的工具，当我们需要加入(N)个集合

(——)我们可以只在查询的顶层应用条件

例子

var Join = require('mongo-join').Join, mongodb = require('mongodb'), Db = mongodb.Db, Server = mongodb.Server;
db.open(function (err, Database) {
    Database.collection('Appoint', function (err, Appoints) {

        /* we can put conditions just on the top level */
        Appoints.find({_id_Doctor: id_doctor ,full_date :{ $gte: start_date },
            full_date :{ $lte: end_date }}, function (err, cursor) {
            var join = new Join(Database).on({
                field: '_id_Doctor', // <- field in Appoints document
                to: '_id',         // <- field in User doc. treated as ObjectID automatically.
                from: 'User'  // <- collection name for User doc
            }).on({
                field: '_id_Patient', // <- field in Appoints doc
                to: '_id',         // <- field in User doc. treated as ObjectID automatically.
                from: 'User'  // <- collection name for User doc
            })
            join.toArray(cursor, function (err, joinedDocs) {

                /* do what ever you want here */
                /* you can fetch the table and apply your own conditions */
                .....
                .....
                .....


                resp.status(200);
                resp.json({
                    "status": 200,
                    "message": "success",
                    "Appoints_Range": joinedDocs,


                });
                return resp;


            });

    });

不，看起来你并没有做错。MongoDB连接是“客户端”。就像你说的

目前，我首先获取符合条件的注释，然后找出结果集中的所有uid，获取用户对象，并将它们与注释的结果合并。看来我做错了。

1) Select from the collection you're interested in.
2) From that collection pull out ID's you need
3) Select from other collections
4) Decorate your original results.

它不是一个“真正的”连接，但它实际上比SQL连接有用得多，因为您不必处理“多”面连接的重复行，而是修饰最初选择的集合。

这一页上有很多废话和FUD。结果5年后，MongoDB仍然存在。

As others have pointed out you are trying to create a relational database from none relational database which you really don't want to do but anyways, if you have a case that you have to do this here is a solution you can use. We first do a foreach find on collection A( or in your case users) and then we get each item as an object then we use object property (in your case uid) to lookup in our second collection (in your case comments) if we can find it then we have a match and we can print or do something with it. Hope this helps you and good luck :)

db.users.find().forEach(
function (object) {
    var commonInBoth=db.comments.findOne({ "uid": object.uid} );
    if (commonInBoth != null) {
        printjson(commonInBoth) ;
        printjson(object) ;
    }else {
        // did not match so we don't care in this case
    }
});

您可以使用聚合管道来实现它，但是自己编写它很麻烦。

您可以使用mongo-join-query从您的查询自动创建聚合管道。

这是你的查询的样子:

const mongoose = require("mongoose");
const joinQuery = require("mongo-join-query");

joinQuery(
    mongoose.models.Comment,
    {
        find: { pid:444 },
        populate: ["uid"]
    },
    (err, res) => (err ? console.log("Error:", err) : console.log("Success:", res.results))
);

您的结果将在uid字段中有user对象，您可以链接任意多的层次。您可以填充对用户的引用，从而引用一个Team，再引用其他东西，等等。

免责声明:我编写了mongo-join-query来解决这个问题。