But now, Swift reportedly feels disrespected by the DJ/producer. A source tells ET that "during their relationship, Taylor wrote the song, sat down at a piano and did a demo into her iPhone."
The Nashville native sent the song to Harris, who apparently loved it. "They both went into a studio and did a full demo with Taylor on vocals and Calvin doing the beat," the source added. "They both knew the song would be a hit, but Taylor wrote it for Calvin and [they] both agreed it was a bad idea to let the world know they collaborated as a couple -- it would overshadow the song."
Swift has since moved on with actor Tom Hiddleston, and the couple has been keeping their whirlwind romance alive by traveling the world.