Set nginx root based on existance of another directory - nginx

Question

I'm building a system which uses dynamic DNS for user accounts, so you register for a sub domain.
I have a few server directives in place to catch things like api. and www. and other special cases, and I have a directive which reads the wildcard domain name and uses it to set a domain specific assets location.
server {
server_name "~^(?<domain>[a-z0-9]+)\.example\.com$";
root /sites/core;
location /assets {
alias /site-assets/$domain;
}
}
What I want to do is check for the existence of the assets directory and present, on the same domain, a different site.
To clarify, if the assets directory in the example above exists, serve one set of files, otherwise, serve another. Since the site I want to serve won't be IN the directory in question, I can't use try_files or anything like that.
I read all the horror stories about using the if directive but I think I need something like
if (-d /site-assets/$domain) {
... Do something
}
Then change the root, but that doesn't work.
Any ideas how I can achieve this?

Answer 1

OK. I figured this out. If there's a problem with it, let me know.
I have the following in a server directive
server {
server_name "~^(?<domain>[a-z0-9]+)\.example\.com$";
set $site core;
if (!-d /path/to/assets/$domain) {
set $site join;
}
root /path/to/sites/$site;
}
If the assets directory for the given domain name does NOT exist, then I can serve up a joining page for my site, which can pick up the domain name and allow the user to register. If it does exist, then serve up the main app.
This seems to me like a simple use of the if directive so shouldn't get me in too much trouble.