XCode Linker error: Symbol(s) not found

Came across a weird error today:

Objc-class-ref-to-[ClassName] in MainController.o
Symbol(s) not found
Collect2: Id returned 1 exit status

For some reason the reference to the class in question is not listed under Compile Sources. Thankfully, it's an easy fix.

1. Under Groups & Files, expand Targets
2. Expand the compile target
3. Expand Compile Sources (xx)
4. Check to see if all of your .m files are listed. If not, drag the missing files in.
5. Recompile and run.

So, not sure why this happens, but luckily there's an easy fix.