"The Middle" was one of the largest collaborative efforts in the pop world (see video below). Zedd spent over a year on the song, trying to find the exact right singer for the job. Luckily he did - the song shot to number 1 with a country singer as the lead vocals!
Here are some of the singers that either passed on the song or weren't chosen to sing on it:
Bebe Rexha (luckily she already had another hit up her sleeve!)
Carly Rae Jepsen
Demi Lovato (she decided it was too pop for the direction she was going on her next album)
Anne-Marie (she was so close to being the final decision that she had artwork and a release date - but already had a song coming out with another producer so Zedd decided against it)