Ellie Goulding and Calvin Harris also appeared
Song is the first to be taken from Lamar's new album, the follow-up to 2012's 'Good Kid, MAAD City'
Rapper included the song in a surprise set at the 'We Day Toronto' educational event
Three-day event culminates in gig on private island in the Bermuda Triangle
The rapper visited the singer's house in St Louis
The Compton rapper unveils the first track from his new studio album
New track 'i' is rumoured to be unveiled next week
Rapper has revealed the single's cover via Twitter
Eric Reed and Willie Jones III of former jazz ensemble Black Note are suing US rapper
Fourteen minute film will be revealed at Next Fest in Los Angeles on August 9