Nested form params do not work reliably

Hi. I think I've just run into a bug in Rails involving nested models and forms. Her's a minimal case.

This works:

<% form_for @recipe do |f| %>
   Ingredient name: <input type="text" name="recipe[ingredient_lines][][ingredient][name]"/>
   <%= submit_tag %>
<% end %>

When form is submitted, params includes

"recipe"=>{"ingredient_lines"=>[{"ingredient"=>{"name"=>"sugar"}}]}

as expected.

This does not work:

<% form_for @recipe do |f| %>
   Unit: <input type="text" name="recipe[ingredient_lines][][unit]"/><br/>
   Ingredient name: <input type="text" name="recipe[ingredient_lines][][ingredient][name]"/>
   <%= submit_tag %>
<% end %>

When form is submitted, params should include

"recipe"=>{"ingredient_lines"=>[{"unit" => "cup", "ingredient"=>{"name"=>"sugar"}}]}

but instead includes

"recipe"=>{"ingredient_lines"=>[{"unit" => "cup", "ingredient"=>{}}]}

This occurs in Rails 2.1.2 and 2.2.2. Let me know if you need more information to track this down. Thanks!