var userSchema = new mongoose.Schema({
    ...
    auth: {
        password: String,
        passToken: String,
        tokenExpires: Date
    },
    ...
});

userSchema.methods.createToken = function(next){
    require('crypto').randomBytes(16, function(err,buf){
        if (err){ next(err); }
        else {
            this.auth.passToken = buf.toString('hex');
            this.auth.tokenExpires = Date.now() + 3600000;
            this.save();
        }
    });
};

/path/to/project/config/models.js:85
    this.auth.passToken = buf.toString('hex');
                        ^

TypeError: Cannot set property 'passToken' of undefined
    at InternalFieldObject.ondone (/path/to/project/config/models.js:85:25)

userSchema.methods.createToken = function(next){
    var user = this;
    require('crypto').randomBytes(16, function(err,buf){
       if (err){ next(err); }
        else {
            user.auth.passToken = buf.toString('hex');
            user.auth.tokenExpires = Date.now() + 3600000;
            user.save();
        }
    });
};