Adding to multiple tables in rails

Assuming that your form passes an author_name and a book hash with the book attributes, you could do the following in your controller:

@author = Author.find_or_create_by_name(params[:author_name])
@book = Book.new(params[:book])
@book.author = @author
@book.save

This will find an author by that name, or create one, then assign that author to the book that's created.

Note: This also assumes that authors can be uniquely identified by their name, and this usually isn't the case. There could be multiple guys namned "Steven King" out there, for instance, and only one of them wrote The Shining

robintw's comment to Micah's answer:

This looks like a good way to do it - although I am surprised in some ways that Rails doesn't do it a bit more automatically

It's four lines! How much more automatically can Rails make it? It's a framework not a human being. :)

And here it is in two:

@author = Author.find_or_create_by_name(params[:author_name])
@book = Book.create params[:book].merge({:author_id => @author.id})

And Honza's answer has a lot of merit:

class Book < ActiveRecord::Base
  ...
  def author_name=(author_name)
    self.author = Author.find_or_create_by_name author_name
  end
end

With this example, and assuming your form has a parameter like "book[author_name]", your code is just:

@book = Book.create params[:book]

Wowzers.

This will eventually be automatic (according to this post, in Rails 2.2): http://ryandaigle.com/articles/2008/7/19/what-s-new-in-edge-rails-nested-models