How to avoid dependency cycles with shared packages?

I know this gets asked a lot but I still don't understand how to fix it. Say I have this structure:

foo
- foos.go      // custom foo type
database
- database.go  // interface
- postgres.go  // implementation

In database.go I need to import foo, so I can saveOnDb(foo.myfoo), and in foos.go I need to import database, so I can database.saveOnDb(myfoo).

This doesn't compile because of the import cycle. What would the correct design be?