MissingSchemaError: Schema hasn't been registered for model “User”

Question

In my models/user.js file:

var mongoose = require(\'mongoose\');
var Schema = mongoose.Schema;

var userSchema = new Schema({
    (define schema         


        

Answer 1

In my case, it happened because of capital/small letter confusion. User model had this:

const userSchema = new Schema({
// ...
});
module.exports = mongoose.model('User', userSchema);

And Product model had a reference to User model but small case:

const productSchema = new Schema({
// ...
  userId: {
    type: Schema.Types.ObjectId,
    ref: 'user', // This should exactly match the name of User model above!!!
    required: true
  }
});