我收到此mongo错误。 'E11000重复密钥错误收集：21day.challenges索引：username_1重复密钥：{：null}'，

Question

每当我添加新的挑战时，我都会收到11000的mongo错误

名称：“ MongoError”，   索引：0，   代码：11000，   errmsg：    'E11000重复密钥错误集合：21day.challenges索引：username_1 dup key：{：null}'，   [Symbol（mongoErrorContextSymbol）]：{}}

我的用户架构是

var mongoose=require("mongoose"),
    passportLocalMongoose=require("passport-local-mongoose");

var userschema= new mongoose.Schema({
//  username:String,
    password:String,
    challenges: [
         {
            type :mongoose.Schema.Types.ObjectId,
            ref  :"challenge"       
    }

    //challengeSchema
    ]
});
userschema.plugin(passportLocalMongoose,{usernameField : "username"});

var user=mongoose.model("user",userschema);
module.exports=user;

挑战模式是

var mongoose= require("mongoose"),
    passportlocalmongoose= require("passport-local-mongoose");

var challengeSchema=new mongoose.Schema({
    text: String,


});
challengeSchema.plugin(passportlocalmongoose);
var challenge=mongoose.model("challenge",challengeSchema);
module.exports=challenge;