Mongoose保存子文档数组

我正在努力保存子文档数组。只是不会保存：

前端发送一个Day对象，该对象有一个标题数组，每个标题都可以有一个内容数组(这是一个mongoose模式(。

var DaySchema = mongoose.Schema({
date: Date,
startTime: Date,
endTime: Date,
title: String,
order: Number,
description: String,
cohort: {
    type: objId,
    ref: 'cohort'
},
instructors: [{
    type: objId,
    ref: 'instructor'
}],
headings: [{
    title: String,
    subTitle: String,
    content: [{
        type:objId,
        ref: 'content'
    }]
}]
});

var ContentSchema = mongoose.Schema({
title: String,
description: String,
contentUrl: String,
dueDate: Date,
dateAssigned: Date,
downloadUrl: String
});

这是我用来PUT和POST一天的代码，以及相关的Content对象。除了保存内容外，一切都正常。当我在mongo shell中查看时，标题下的每个内容字段都是这样的：

content: []

有什么想法吗？

exports.putdaycontent = function (req, res) {
var dayId = req.params.dayId;
var headings = req.body.headings;
var day = req.body;
var updateDay = function () {
    Day.update({_id: dayId}, day, {new: true, upsert: true}, function (err, day) {
        if (err)
            error.databaseError(req, res, err);
        res.send(day);
    });
};
if (headings) {
    for (var x = 0; x < headings.length; x++) {
        var h = headings[x];
        if (h.content) {
            for (var y = 0; y < h.content.length; y++) {
                var c = h.content[y];
                //If existing content update it
                if (c._id && c._id.length > 0) {
                    Content.update({_id: c._id}, c, {new: true,upsert: true}, function (err, content) {
                    });
                } else {
                    var content = new Content(c);
                    h.content[y] = content;
                }
            }
        }
    }
    day.headings = headings;
    updateDay();
} else {
    updateDay();
}

};
exports.postdaycontent = function (req, res) {
var headings = req.body.headings;
var day = req.body;
var cohortId = day.cohort._id;
var postDay = function () {
    var d = new Day(day);
    console.log("CONTENT DAYS:",d);
    d.save(function(err, newDay) {
       if (err)
           error.databaseError(req, res, err);
        Cohort.findOneAndUpdate({_id: cohortId}, {$push: {days: newDay}}, {upsert: true}, function(err, newCohort) {
            res.send({msg:"Successfully saved day to cohort"});
        });
    });
};
if (headings) {
    for (var x = 0; x < headings.length; x++) {
        var h = headings[x];
        if (h.content) {
            for (var y = 0; y < h.content.length; y++) {
                var c = h.content[y];
                var content = new Content(c);
                h.content[y] = content;
                console.log("CONTENT:",content);
            }
        }
    }
    day.headings = headings;
} 
postDay();
};