MongoDB save（）问题与对象数组

Question

This code is a post request for sending messages between users. It succeeds when the message is not part of an existing message string (ie: typeof existingMessageIndex === 'undefined' ). The docs.save() results in the error ( "error saving outgoing message" when existingMessageIndex is not undefined.

It appears that mongo doesn't like that I'm trying to save an array of objects as one item in the existing array field.

Anyone know what I'm missing?

.post(function(req,res,next){
    var messageStringTitle = req.body.originatingTitle !== ''?req.body.originatingTitle:req.body.messageTitle;

    var newMessage = {
        senderDisplay: req.user.displayname,
        senderUser: req.user.username,
        senderImage: req.user.profileimg,
        originatingTitle: messageStringTitle,
        title: req.body.messageTitle,
        content: req.body.messageContent
    };
    var messageSender = function(docs){
        var tempMessageArray = docs.messagein;

        for(var i = 0; i<tempMessageArray.length; i++){
            console.log("Loop has executed: " + i);
            if(tempMessageArray[i][0]){
                if(tempMessageArray[i][0].originatingTitle===messageStringTitle && tempMessageArray[i][0].senderUser===req.user.username || tempMessageArray[i][0].senderUser===req.body.messageUser){
                    var existingMessageIndex = i;
                }
            }
        }

        console.log("===================" + existingMessageIndex);
        if(typeof existingMessageIndex === 'undefined'){
            tempMessageArray.unshift([newMessage]);
            docs.messagein = tempMessageArray;
        }else{
            docs.messagein[existingMessageIndex].unshift(newMessage);
        }
        return docs;
    };


    //Update sending user
    User.findOne({username: req.user.username}, function(e, docs){
        messageSender(docs);
        console.log(docs);   //Logs exactly what I want to be saved, however saving fails
        docs.save(function(e){
            console.log("Error saving outgoing message");
        })
    });


    //Update receiving user
    User.findOne({username: req.body.messageUser}, function(e, docs){
        messageSender(docs);
        console.log(docs);
        docs.save(function(e){
            if(e){
                console.log("Error sending message within post ('/messages')");
            }
        });

    });

Answer 1

Solution: Need to use db.collection.markModified() because the schema 'messagein' was changed to type mixed when it became an array of arrays of objects.

 //Update sending user
    User.findOne({username: req.user.username}, function(e, docs){
        messageSender(docs);
        console.log(docs);   //Logs exactly what I want to be saved, however saving fails
        docs.markModified('messagein'); //ADDED
        docs.save(function(e){
            console.log("Error saving outgoing message");
        })
    });


    //Update receiving user
    User.findOne({username: req.body.messageUser}, function(e, docs){
        messageSender(docs);
        console.log(docs);
        docs.markModified('messagein'); //ADDED
        docs.save(function(e){
            if(e){
                console.log("Error sending message within post ('/messages')");
            }
        });

Note: The solution worked, however the explanation is just my guess, please correct me if I'm wrong. Thanks!