如何使用mongoose检查记录/文档是否已存在于MongoDB？

Question

I am trying to check if a record exists in MongoDB using mongoose. For that I am using the function findOne(). But even if the record/document does not exists the query returns a non null value. How to use this function or any other way to check if a document exists? My code is:

var req_username = "";
var req_password = "";
const insertUserIntoDatabase = (requestBody) => {
    req_username = requestBody.username;
    req_password = requestBody.password;
    connectToDatabase();
    if (doesUserExistAlready()==true) {
      console.log("user already there");
    }else{
      insertUser();
    }
}                                              

const connectToDatabase = () => {
  mongoose.connect("mongodb://localhost/my_database",
  {useNewUrlParser:true});
}

const doesUserExistAlready = () => {
  const doc = user.findOne({username:req_username});
  if (doc == null) {
    return false;
  }else{
    return true;
  }
}

const insertUser = () => {
  var newUserDoc = new user();
  newUserDoc.username = req_username;
  newUserDoc.password = req_password;
  newUserDoc.save();
}

Answer 1

This is because mongoose 's findOne returns a "Mongoose document", which is essentially a glorified wrapper. this means if it's a null value it will still have the "mongoose document" properties.

You want to be using the lean option to bypass this:

const doc = user.findOne({username:req_username}).lean();

Now if the user does not exist doc will have a null value as expected.

Answer 2

Try this:

const connectToDatabase = () => {
    mongoose.connect("mongodb://localhost/my_database", {
        useNewUrlParser: true
    });
}
var req_username = "";
var req_password = "";
user.findOne({
        username: req_username
    })
    .then(async (user) => {
        if (user) {
            console.log("user already there");
        } else {
            var newUserDoc = new user();
            newUserDoc.username = req_username;
            newUserDoc.password = req_password;
            await newUserDoc.save();
        }
    })

Answer 3

Mongoose has a Model.exists() method which returns true if the record exists, or false otherwise. For instance:

// on your user models file which is inside Models folder
const mongoose = require("mongoose");

let UserSchema = new mongoose.Schema({
  name: { "John Doe", required: [true, "Please provide a name"] }
})
module.exports = mongoose.model('User', UserSchema);

Then in your controllers, import the User from your model like so

const User = require('../models/User');

let checkUser = await User.exists({ name: "John Doe" });
console.log(checkUser) // true
// If you check for another name that does not exist, it will return false

Read more here: https://mongoosejs.com/docs/api.html#model_Model-exists