Mongoose: Find, modify, save

I have a Mongoose User model:

var User = mongoose.model('Users',
    mongoose.Schema({
        username: 'string',
        password: 'string',
        rights: 'string'
    })
);

I want to find one instance of the User model, modify it's properties, and save the changes. This is what I have tried (it's wrong!):

User.find({username: oldUsername}, function (err, user) {
    user.username = newUser.username;
    user.password = newUser.password;
    user.rights = newUser.rights;

    user.save(function (err) {
        if(err) {
            console.error('ERROR!');
        }
    });
});

What is the syntax to find, modify and save an instance of the User model?

The user parameter of your callback is an array with find. Use findOne instead of find when querying for a single instance.

User.findOne({username: oldUsername}, function (err, user) {
    user.username = newUser.username;
    user.password = newUser.password;
    user.rights = newUser.rights;

    user.save(function (err) {
        if(err) {
            console.error('ERROR!');
        }
    });
});

Why not use Model.update? After all you're not using the found user for anything else than to update it's properties:

User.update({username: oldUsername}, {
    username: newUser.username, 
    password: newUser.password, 
    rights: newUser.rights
}, function(err, numberAffected, rawResponse) {
   //handle it
})

If you want to use find, like I would for any validation you want to do on the client side.

find returns an ARRAY of objects

findOne returns only an object

Adding user = user[0] made the save method work for me.

Here is where you put it.

User.find({username: oldUsername}, function (err, user) {
    user = user[0];
    user.username = newUser.username;
    user.password = newUser.password;
    user.rights = newUser.rights;

    user.save(function (err) {
        if(err) {
            console.error('ERROR!');
        }
    });
});