Why does my code throw this exception- MissingSchemaError: Schema hasn't been registered for model "User".?

Question

Problem: I was trying to get my head around on population in mongoose.js, I got this error out of nowhere when I tried to populate the 'user' field of a post document.

PS: This is complete code except for the connection string part, so you can try and see what went wrong. Thanks in advance!!!

const express = require('express');
const mongoose = require('mongoose');

//set up the server
const app = express();
app.listen(3000);
console.log("Server listening to port 3000");

//set up the mongosoe connection
mongoose.connect("YOUR MONGODB URI", {
  useCreateIndex : true,
  useNewUrlParser : true,
  useUnifiedTopology : true,
}).then(() => {
  console.log("Connection established");
}).catch(err => {
  throw err;
});

//set up the models
const User = mongoose.model('user', new mongoose.Schema({
  name : String,
}));

const Post = mongoose.model('post', new mongoose.Schema({
  user : {
    type : mongoose.Schema.Types.ObjectId,
    ref : 'User',
  },
  content : String,
}));

//create a User and Post
new User({
  name : "Nike",
}).save((err, user) => {
  console.log("The user was saved");
  new Post({
    user : user._id,
    content : "I laugh a lot",
  }).save(() => {
    console.log("The post is saved");
  });
});


//populate the post with user
(async function() {
  try {
    let post = await Post.findOne({}).populate('user');
    console.log(post);
  } catch (err) {
    throw err;
  }
})();

Error

(node:41792) UnhandledPromiseRejectionWarning: MissingSchemaError: Schema hasn't been registered for model "User".
Use mongoose.model(name, schema)
    at NativeConnection.Connection.model (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\connection.js:1192:11)
    at getModelsMapForPopulate (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\helpers\populate\getModelsMapForPopulate.js:285:59)
    at populate (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\model.js:4320:21)
    at _populate (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\model.js:4290:5)
    at C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\model.js:4266:5
    at promiseOrCallback (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\helpers\promiseOrCallback.js:9:12)
    at Function.Model.populate (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\model.js:4264:10)
    at model.Query.Query._completeOne (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\query.js:2109:9)
    at Immediate.<anonymous> (C:\Users\Nikesh K Lepcha\Documents\node_modules\mongoose\lib\query.js:2148:10)
    at Immediate.<anonymous> (C:\Users\Nikesh K Lepcha\Documents\node_modules\mquery\lib\utils.js:116:16)
    at processImmediate (internal/timers.js:456:21)

Answer 1

You registered user but reference it as User . Look at the difference:

mongoose.model('user'

and

ref : 'User'