search for: category_location

I have two tables and a join table for them e.g. books, authors in a many to many relationship (habtm) and a join table books_authors. I can successfully search for a book that has "author.id = 2 OR author.id = 4" but I am unable to search for "author.id = 2 AND author.id = 4" This is because the result of all the joins only has one author.id column so no single row has