John, a house painter, enters into a contract with Robert under which he promises to paint Robert's house and Robert promises to pay John $1,000.00 upon completion of the work. John mistakenly paints the house next door to John, which belongs to Mary. Mary returns from vacation and is surprised to see her house beautifully painted. John asks Mary to pay for the work. Mary refuses. Under what theory may John attempt to recover from Mary? Who will prevail - John or Mary?