有一个很多驱动程序都支持的规范叫做DBRef。

DBRef是用于在文档之间创建引用的更正式的规范。DBRefs(通常)包括集合名称和对象id。大多数开发人员只在集合可以从一个文档更改到下一个文档时才使用DBRefs。如果您引用的集合总是相同的，那么上面概述的手动引用更有效。

摘自MongoDB文档:数据模型>数据模型参考> 数据库的引用

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
    }
});

在3.2.6之前，Mongodb不像mysql那样支持join查询。下面是适合你的解决方案。

 db.getCollection('comments').aggregate([
        {$match : {pid : 444}},
        {$lookup: {from: "users",localField: "uid",foreignField: "uid",as: "userData"}},
   ])

查找美元(聚合)

对同一数据库中的未分片集合执行左外连接，以从“已连接”集合中筛选文档进行处理。$查找阶段向每个输入文档添加一个新的数组字段，其元素是“已加入”集合中的匹配文档。$查找阶段将这些重新塑造的文档传递给下一个阶段。 $查找阶段的语法如下:

平等的比赛

要在输入文档中的字段与" joined "集合中的文档中的字段之间执行相等匹配，$lookup stage的语法如下:

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

该操作将对应于以下伪sql语句:

SELECT *, <output array field>
FROM collection
WHERE <output array field> IN (SELECT <documents as determined from the pipeline>
                               FROM <collection to join>
                               WHERE <pipeline> );

蒙哥URL

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。

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;


            });

    });