Why I can't use ctx.body in the callback of save()?

Question

I can use ctx.body in find() 's callback:

router.post("/register/isNameUsed", async (ctx, next) => {
    let username = ctx.request.body.username;
    await userInfo.find({ username: username }, function(err, doc) {
        if (err) {
            console.log(err);
        } else {
            if (doc.length > 0) {
                ctx.body = { isNameUsed: true };
            } else {
                ctx.body = { isNameUsed: false };
            }
        }
    });

    await next();
});

But I can't use it in save() 's callback:

router.post("/register", async (ctx, next) => {
    let username = ctx.request.body.name;
    let password = ctx.request.body.password;
    var ui = new userInfo({
        username,
        password
    });
    await ui.save(function(err, doc) {
        if (err) {
            console.log(err);
        } else {
            ctx.body = { registerSuccess: true };//It doesn't work
        }
    });
    await next();
});

The code runs successfully，just the ctx.body doesn't work, why?

Answer 1

OK，I changed my code to this:

router.post("/register", async (ctx, next) => {
    let username = ctx.request.body.name;
    let password = ctx.request.body.password;

    var ui = new userInfo({
        username,
        password
    });

    try {
        let insertRes = await ui.save();
        ctx.body = { registerSuccess: true };
    } catch (error) {
        ctx.body = { registerSuccess: false };
    }

    await next();
});

Then the ctx.body worked.

And I'll never write ctx.body in the callback, it's so queasily...

But I still don't know why the ctx.body can work in the find() 's callback but not in the save() s?