问如何在MongoDB中用mongoose将多个引用保存到一个文档中？

EN

var tagSchema = mongoose.Schema({
    tagname: String,
});

var tag = mongoose.model('Tag', tagSchema);


var Item = new Schema({
    title : String,
    description : String,
    price : Number,
    tags:  [{
        type: mongoose.Schema.Types.ObjectId,
        ref: "Tag"
    }]
});

var product = mongoose.model('Product', Item);

product.create(req.body.product, function(err, newitem){
            if(err){
                console.log(err);
            }else{

             console.log(req.body.tags); // gives ["fun","cheap"] from the form
             req.body.tags.forEach(function(newtag){
             tag.findOne({tagname : newtag},function(err, finalcast){


             newitem.tags.push(finalcast); 
             console.log(newitem); // gives the product with the tag collections in it.

               });
              newitem.save(); // error Can't save() the same doc multiple times in parallel.
              console.log(newitem); // the product collection has an empty tags[] 

               });


                 res.redirect("/");

            }

        });