Question

I have to following code:

var UserSchema = mongoose.Schema({
  email: {
    type: String,
    unique: true
  },
  username: {
    type: String,
    unique: true
  }
});

var User = db.model('User', UserSchema);

var user = new User();

user.email = '[e-mail]'
user.username = 'peter'

user.save(function(err){
  if (err) return console.log(err)
  console.log('user saved')
})

On the first run it shows user saved which is totally fine. On the second run it gives me

{ [MongoError: E11000 duplicate key error index: node-login.users.$username_1  dup key: { : "peter" }]
  name: 'MongoError',
  err: 'E11000 duplicate key error index: node-login.users.$username_1  dup key: { : "peter" }',
  code: 11000,
  n: 0,
  lastOp: 0,
  connectionId: 1455732,
  ok: 1 }

What about the email? It is also a duplicate and should return an error. If only one error is thrown each time, why is it the username and not email? email is above username in the schema. How does mongoose decide about what duplicate to check/return first?

Answer 1

It’s not Mongoose that’s doing this, it’s MongoDB. Which one of the two duplicate fields that’s flagged is likely undefined. I assume it’s simply throwing the error as soon as the first index it checks fails the uniqueness test.

If you need more information than what’s provided in the error, you’ll need to query for all potential duplicate fields separately.