默认的mongoose _id没有增加

Question

默认_id:ObjectId不会自动递增，这是正常的吗？

MongoDB：2.2.3 猫鼬：3.5.8

架构我尝试保存：

var userSchema = mongoose.Schema({
  username: { type: String, required: true, unique: true },
  email: { type: String, required: true, unique: true },
  password: { type: String, required: true},
});
var User = mongoose.model('User', userSchema);

保存已创建的模型：

var newUser = new User({email:login, password:password, username:username});
newUser.save();

服务器错误：

{ email: 'joe@joe.com',
  password: 'joe',
  username: 'joe',
  _id: 51444c0dcc6b4e040c000001 }
{ [MongoError: E11000 duplicate key error index: supinfarm.users.$login_1  dup key: { : null }]
  name: 'MongoError',
  err: 'E11000 duplicate key error index: supinfarm.users.$login_1  dup key: { : null }',
  code: 11000,
  n: 0,
  connectionId: 431,
  ok: 1 }

db.users.find（）：

{
  "email" : "c@c.com", "password" : "hereTheHash",
  "username" : "canard", "__v" : 0,
  "_id" : ObjectId("514447c805a669d40b000001")
}