merge() is not a function

[dunklesToast]

Bug report

When I want to merge an document, it will throw an error :/

My Code:

user.save((err, doc) => {
    if(err) throw err;
    fs.readFile('./filter.png', (err, data) => {
        const dataToAdd = {
            profilePicture: data
        };
        doc.merge(dataToAdd).then((result) => {
            console.log(result)
        })
    })
});

Error Message

/Users/tom/WebstormProjects/<ProjectName>/test.js:28
        doc.merge(dataToAdd).then((result) => {
                             ^

TypeError: doc.merge(...).then is not a function
    at fs.readFile (/Users/tom/WebstormProjects/<ProjectName>/test.js:28:30)
    at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:503:3)

Question

Am I doing something wrong?
The Usermodel is defined in another JS Document which is required. Thinky or RethinkDBDash arent required in the file where I want to merge.

Well, this is just a simplified version of what I want to do - I know it makes no sense to merge directly after creation. But in my "real" case I want to merge after I created the user, but the way I want to do it is mostly the same

[dunklesToast]

I found my mistake.
It needs to be
user.merge().save().then((result) => {})
I missed that save()