mongodb - adding multiple same documents using addtoset command in mongoose -

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my schema 

var userquizschema = mongoose.schema({            uid:{type:objectid,required: true,index:true},         answer:[{content:string, qid:objectid ,time:date }],         });

in schema, 'uid' represents user identifier, while 'answer' array stores answers student had answered. in each answer, qid relates question id, , 'content' student's real answer, 'time' modified time stamp answer.

here use mongoose upsert new answers array 

function updateanswer(uid,question_id,answer,callback){      var options = { new: false };        var quiz_id = mongoose.types.objectid(quiz_id);     var qid = mongoose.types.objectid(question_id);     userquizmodel.findoneandupdate({'uid':uid},{'$addtoset':{'answer':{'qid':qid, 'content':answer} } },options,function(err,ref){         if(err) {             console.log('update '.red,err);             callback(err, null);         }else{             console.log('update '.green+ref);                              callback(null,ref);         }     }) }

in common sense, using addtoset command, element in answer array should unique, in example, answer array have multiple same embedded documents except each embedded document has 1 unique ojbectid _id

such 

 answer:   [ { qid: 5175aecf0e5b061414000001, _id: 518a5e5895fc9ddc1e000003 },     { qid: 5175aecf0e5b061414000001, _id: 518a5e5f95fc9ddc1e000004 } ] }

you see qid of 2 embedded documents same, _id different.

why there additional _id, don't put schema design ??

you can disable _id in embedded objects explicitly defining schema elements _id option set false:

var userquizschema = mongoose.schema({        uid:{type:objectid,required: true,index:true},     answer:[new schema({content:string, qid:objectid, time:date}, {_id:false})] });

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