title: Do a certain backend operation on clicking Django Admin save button
body: <p>In a library system, I have the Userbooks (refers to books issued by a user) object registered with Django Admin. And when Django Admin creates a Userbooks object and saves it, the Book object (refers to books in library, also registered with Django Admin) associated with that UserBook with a one_to_one relationship, needs to have its boolean field 'is_issued' to be set to true. How can I do this backend action when Admin clicks the 'save' button?</p>