I recently purchased The Forge DLC as it has been advertised as being "Standalone", however it turns out this isn't the case. I even checked the description on the PS store before buying as I believe every DLC that isn't standalone states something along the lines of it requiring a copy of the base game to play, but yet nowhere does it actually state this.
Anyway after downloading and discovering this I then contacted Sony to request a refund but was told that because I've already downloaded it I am therefore ineligible to receive a refund. I did already own the base game but have since sold it, so it looks like I'll have to rebuy the game.
Just thought I'd post this to warn anyone that was thinking of purchasing the DLC and are in the same situation of no longer owning the game anymore.