It would be nice to supply an array to fields_for when working with nested attributes, like so:

# In controller:
@selected_books = @author.books.all(:conditions => {:selected => true})

View:

form_for :author do |f|
  f.text_field :name
  f.fields_for(:books, @selected_books) do |f2|

f2.text_field :name

  end
end

This is instead of iterating over the array as currently shown in the docs:

form_for :author do |f|
  ...
  @selected_books.each do |book|

f.fields_for(:books, book) do |f2|
  ...
end

  end
end

Supplying an array seems more elegant to me and stops the parent object association being called for every iteration.

The supplied patch passes the existing tests and adds a couple more that are appropriate. Although the nested if statements are a bit messy so any help making it cleaner would be welcome.