find_by() returns a single instance, not an array.
Movie.find_by(id:3).title will return the title of the movie with id 3. if the id doesnt exist, it returns "nil".
find() also returns a single instance, but assumes you are searching for primary key (ID).
Movie.find(3).title returns the title of movie with id 3. if id doesnt exist, it returns 404 error.
