MEAN 2 Multi Tenancy (Multiple collection saves that reference each other) - node.js

Question

So whilst learning JS and specifically the MEAN 2 stack i'm trying to build out a basic multi tenanted app. Im building out sign up routes in express and the flow i'm trying to achieve would be:
Sign up with company name, email and password. The info would go to save a new tenant, then return the _id of the new tenant and then use this new id, the email and the password to save a new user.
The closest is:
router.post('/', function (req, res, next) {
var tenant = new Tenant({
name: req.body.name
});
var newTenant;
tenant.save(function (err, tenant) {
if (err) {
return res.status(500).json({
title: 'An error has occured',
error: err
});
}
res.status(201).json({
message: 'Tenant created',
obj: tenant
});
return(tenant._id);
newTenant = tenant;
});
Tenant.findById(newTenant._id, function(err, tenant) {
if (err) {
return res.status(500).json({
title:'An error occured',
error: err
});
}
var user = new User({
email: req.body.email,
password: bcrypt.hashSync(req.body.password, 10),
active: req.body.active,
tenant: tenant
});
user.save(function (err, user) {
if (err) {
return res.status(500).json({
title: 'An error has occured',
error: err
});
}
res.status(201).json({
message: 'User created',
obj: user
});
});
});
});
module.exports = router;
I'm getting an error: cant set headers after they've been sent.
I think I know where i'm wrong, with returning the tenant info. I think Async is the answer but cant figure out how to implement it. Sorry if this is a stupid question or i'm missing something obvious, I'm super new to this and callbacks are doing my head in.

Answer 1

This is happening because res.status() sets headers as soon as it has fired. You try to do this multiple times both when checking for errors, and then you try to set the status code again in Tenant.findById().
You end up with a flow like:
if (err) set headers
set headers (again)
findById()
if (err) set headers
set headers (again)
You have to be careful when writing out your response that you only do it at the last point in your logic flow. You can also could set up a global err handler and throw new Error() and stop the flow of logic and handle the output immediately. If you don't, your code will continue to execute even though it encountered an error.
Another tip: callbacks don't work well with returns. And although you can arrange them to work, or implement a promise architecture instead, the simplest fix (and the easiest to learn) is to make your functions all asynchronous.
Try instead something like:
tenant.save(function (err, tenant, callback) {
// add a callback param to your fn ^
if (err) {
throw({
code: 500,
title: 'An error has occured',
error: err
});
} else {
// ^ add an else statement so you don't set the headers twice
// (because res.status() sets headers)
res.status(201).json({
message: 'Tenant created',
obj: tenant
});
}
callback(err, tenant);
// call your async function instead of return,
// and pass both err and tenant as params
// (one or the other may be undefined though, if it fails/succeeds)
});
... Create additional isolated functions (or even modules) for the rest of your tasks, then you can then call your function like this:
tenant.save(function(err, tenant) {
Tenant.findById(tenant._id, function(err, tenant) {
var user = new User({...})
user.save()
});
});