Not able to initialize and push values to array using mongoose.

I have to initialize an array to [](empty array) and then push user chose "tags" into it. This is my schema:

 var user_schema = new Schema({
   username:{type:String,required: true, unique : true, trim: true},
   college:{type:String,required: true},
   password:{type:String,required: true, trim: true},
   email:{type:String,required: true, unique : true, trim: true},
   phone:{type:Number,required: true, unique : true, trim: true},
   dp:{type:String},
   tags:[{type:String}],
   description:{type:String},
   skills:{type:String},
   bucket:[{type:String}]
 }); 

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` and i have to initialize "tags" to [] and push user string values (user entered/chosen) every time the request is made to the API call.

I am using this line of code:

 stu_user.findOneAndUpdate(
   { _id: req.decoded.id },
   { $set:{tags:[]},$push: { tags: 'some chosen string'  } },
  function (error, success) {
         if (error) {
            console.log(error);
        } else {
            console.log(success);
        }
    });

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But it's not working.