如何更改mongodb中存在的字段的数据类型

Question

我目前正在使用带有mongoose的nodejs来更新mongodb文档。但是，我试图将字段从Date类型更改为String类型。我认为这会很简单，但这显然不起作用......

function loopThroughDocs(docs) {
    var x = 1;
    docs.forEach(function(i) {

        //sendNewDate and id for update in db

        var dateFormat = formatDate(i.releaseDate);
        updateProjWithStringDate(i, dateFormat);
        x++;
    });

    if(x > docs.length) {
        console.log("Finished going through docs");
    }


}

function formatDate(theDate) {
        var dateObj = new Date(theDate);
        var month = dateObj.getUTCMonth(); //months from 1-12
        var day = dateObj.getUTCDate();
        var year = dateObj.getUTCFullYear();


        var monthsName = ["January", "February", "March", "April", "May", "June", "July", 
        "August", "September", "October", "November", "December"];

        if(year < 2016) {
            console.log("Date is blank _____________");
            return "";
        } else {
            console.log("Date is originally " + theDate);
            return monthsName[month] + " " + day + ", " + year;
        }
}
function updateProjWithStringDate(proj, stringDate) {
    Project.update({_id: proj._id}, {releaseDate: stringDate}, function(err, res) {
        if(err) {
            console.log("Error " + err);
        } else {
            console.log("Updated Date for " + proj.title + " the date is " + stringDate);
        }
    });
}

数据如下所示，不会更改为字符串值：

"releaseDate": {
        "$date": "2017-01-01T05:00:00.000Z"
    },

现在我想弄清楚如何将日期更改为字符串...有一件事是我需要原始日期，因为我使用它来创建字符串，在我创建字符串后，我可以用日期替换日期字符串。