Mongo中没有“joins”。但是你要做的是改变你的用户模式，将评论和帖子文档的ObjectId存储在你的用户数组中。然后在你需要用户数据时使用“populate”。

const userSchema = new Schema({  
    nick_name:{type:String},  
    email: {  
        type: String,  
        trim: true,  
        required: '{PATH} is required!',
        index: true,
    },
    comments: [{ type: Schema.Types.ObjectId, ref:'Comment' }],
    posts: [{ type: Schema.Types.ObjectId, ref:'Post' }]
}, {timestamps: true});

mongoose.model('User', userSchema);

您的查询将如下所示：

User.find()
    .populate('comments posts') // multiple path names in one requires mongoose >= 3.6
    .exec(function(err, usersDocuments) {
        // handle err
        // usersDocuments formatted as desired
    });

Mongoose populate docs

这是可能的。您应该使用聚合。它应该工作。初始化变量

var mongoose = require('mongoose');
    var userCollection = require('./user');//import user model file
    var resources = {
    nick_name: "$nick_name",
    email: "$email"};

    userCollection.aggregate([{
            $group: resources
        }, {
            $lookup: {
                from: "Comments", // collection to join
                localField: "_id",//field from the input documents
                foreignField: "user_id",//field from the documents of the "from" collection
                as: "comments"// output array field
            }
        }, {
            $lookup: {
                from: "Post", // from collection name
                localField: "_id",
                foreignField: "user_id",
                as: "posts"
            }
        }],function (error, data) {
         return res.json(data);
     //handle error case also
});

当然这是可能的，你只需要使用populate，让我告诉你如何：
导入模式

var mongoose = require('mongoose');
var userSch = require('userSchema');
var postSch = require('postSchema');
var commSch = require('commentSchema');

初始化所有必要的变量

var userModel = mongoose.model('User', userSch);
var postModel = mongoose.model('Post', postSch);
var commModel = mongoose.model('Comment', commSch);

现在，执行查询

postModel.find({}).populate('User')
    .exec(function (error, result) {
        return callback(null, null);
    });

commModel.find({}).populate('User')
    .exec(function (error, result) {
        return callback(null, null);
    });

通过这种方式，你可以让用户了解你的评论和帖子，为了让帖子和评论了解用户，你必须执行3个查询，一个针对用户，一个针对评论，一个针对帖子，然后将所有查询混合在一起

你可以考虑在User模型中使用populate virtual来填充评论和帖子，比如docs（https：//mongoosejs.com/docs/populate.html#populate-virtuals）。User.find({filter}).populates('virtualComments').populate('virtualPosts')

除了以上回答之外：**如果我们只想为填充的文档返回几个特定的字段，该怎么办？**这可以通过将常用的字段名语法作为第二个参数传递给populate方法来实现：

Story.
  findOne({ title: /casino royale/i }).
  populate('author', 'name'). // only return the Persons name
  exec(function (err, story) {
    if (err) return handleError(err);

    console.log('The author is %s', story.author.name);
    // prints "The author is Ian Fleming"

    console.log('The authors age is %s', story.author.age);
    // prints "The authors age is null"
  });

参考：https://mongoosejs.com/docs/populate.html#field-selection

使用聚合可以在单个查询中完成工作，这几乎与连接类似。

var mongoose = require('mongoose');
var userModel = require('./user');//import user model file
let result = await userModel.aggregate([
  {
    $match: {
      user: mongoose.Types.ObjectId(req.body.user_id),//pass the user id
    }
  },
  {
    $lookup: {
      from: "comments",//your schema name from mongoDB
      localField: "_id", //user_id from user(main) model
      foreignField: "user_id",//user_id from user(sub) model
      pipeline: [ 
        {
          $project:{ //use to select the fileds you want to select
            comment:1, //:1 will select the field
            is_active :1,
            _id:0,//:0 will not select the field
          }
        }
      ],
      as: "comments",//result var name
    }
  },
   {
    $lookup: {
      from: "post",//your schema name from mongoDB
      localField: "_id", //user_id from user(main) model
      foreignField: "user_id",//user_id from user(sub) model
       pipeline: [
        {
          $project:{//use to select the fileds you want to select
            post:1,//:1 will select the field
            is_active :1,
              _id:0,//:0 will not select the field
          }
        }
      ],
      as: "posts",//result var name
    }
  },
  {
    $project:{//use to select the fileds you want to select
      nick_name:1,//:1 will select the field
      email:1,
      _id:0,//:0 will not select the field
      comments:1,
      posts:1
    }
  }
])