This one is on
this one is on (one)
This drink, meal, performance, etc., is going to be paid for by one. A: "How much do I owe you for the tickets?" B: "Nothing, this one's on me." No, no, this one is on the house—the boss insists.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
This one is on someone
the cost of this drink or meal will be paid by the person named or designated. A: Should we ask for separate checks or what? B: No, no, this one's on the boss! As the waiter set down the glasses, Fred said, "This one's on me."
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.