Union-Find link-by-rank preserve a root

Question

Suppose I have two union-find trees with roots $x$ and $y$ respectively. I want to join them in constant time (this is normally possible since I already "hold" the roots) but I need $x$ to be the root of the merged tree. I don't know what rank $x$ or $y$ have (or more precisely formulated I need this to work always - for any $x$ and $y$ I want the final tree to have $x$ as its root). Is that possible? I couldn't find a solution to this, but it's really not my day today :)