Include only the latest/newest associated record with active record?

This is a working scope with lambda to pass parameters to the scope to select the genre id.

scope :latest_with_genre, lambda do |searched_genre_id|
  joins(:books)
    .where('books.date_of_publication = (SELECT MAX(books.date_of_publication) FROM books WHERE books.author_id = authors.id)')
    .where("books.genre_id = #{searched_genre_id}").group('author.id')
end

This answer Rails query through association limited to most recent record? from Pan Thomakos helped me for the scope.
This answer Pass arguments in scope from keymone helped me for passing argument

I guess this should work

class author
  attr_accessible :first_name, :last_name, :birthday
  has_many :books

  scope :latest, joins(:books).where('books.author_id = authors.id').order('date_of_publication DESC').group('authors.id')
end

Update

For your updated question,you need a scope like this in Book model.

class book
  attr_accessible :pages, :date of publication, :title, _genre_id
  belongs_to :author
  belongs_to :genre

  scope :latest_with_genre, joins(:author,:genre).where("author_id =?","genre_id =?,author.id,genre.id).order('date_of_publication DESC').group('author.id','genre.id')

end

This should work i guess.