Fix nested attributes with specified collection.

actionpack/lib/action_view/helpers/form_helper.rb

@@ -1172,7 +1172,9 @@ def nested_attributes_association?(association_name)
 
         def fields_for_with_nested_attributes(association_name, args, block)
           name = "#{object_name}[#{association_name}_attributes]"
-          association = args.first.to_model if args.first.respond_to?(:to_model)
+          options = args.extract_options!
+          association = args.shift
+          association = association.to_model if association.respond_to?(:to_model)
 
           if association.respond_to?(:new_record?)
             association = [association] if @object.send(association_name).is_a?(Array)
@@ -1181,20 +1183,22 @@ def fields_for_with_nested_attributes(association_name, args, block)
           end
 
           if association.is_a?(Array)
-            explicit_child_index = args.last[:child_index] if args.last.is_a?(Hash)
+            explicit_child_index = options[:child_index]
             association.map do |child|
-              fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name)}]", child, args, block)
+              fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name)}]", child, options, block)
             end.join
           elsif association
-            fields_for_nested_model(name, association, args, block)
+            fields_for_nested_model(name, association, options, block)
           end
         end
 
-        def fields_for_nested_model(name, object, args, block)
+        def fields_for_nested_model(name, object, options, block)
+          object = object.to_model if object.respond_to?(:to_model)
+
           if object.new_record?
-            @template.fields_for(name, object, *args, &block)
+            @template.fields_for(name, object, options, &block)
           else
-            @template.fields_for(name, object, *args) do |builder|
+            @template.fields_for(name, object, options) do |builder|
               block.call(builder)
               @template.concat builder.hidden_field(:id) unless builder.emitted_hidden_id?
             end

actionpack/test/template/form_helper_test.rb

@@ -918,6 +918,28 @@ def test_nested_fields_for_with_existing_records_on_a_supplied_nested_attributes
     assert_dom_equal expected, output_buffer
   end
 
+  def test_nested_fields_for_with_existing_records_on_a_supplied_nested_attributes_collection_different_from_record_one
+    comments = Array.new(2) { |id| Comment.new(id + 1) }
+    @post.comments = []
+
+    form_for(:post, @post) do |f|
+      concat f.text_field(:title)
+      f.fields_for(:comments, comments) do |cf|
+        concat cf.text_field(:name)
+      end
+    end
+
+    expected = '<form action="http://www.example.com" method="post">' +
+               '<input name="post[title]" size="30" type="text" id="post_title" value="Hello World" />' +
+               '<input id="post_comments_attributes_0_name" name="post[comments_attributes][0][name]" size="30" type="text" value="comment #1" />' +
+               '<input id="post_comments_attributes_0_id" name="post[comments_attributes][0][id]" type="hidden" value="1" />' +
+               '<input id="post_comments_attributes_1_name" name="post[comments_attributes][1][name]" size="30" type="text" value="comment #2" />' +
+               '<input id="post_comments_attributes_1_id" name="post[comments_attributes][1][id]" type="hidden" value="2" />' +
+               '</form>'
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_nested_fields_for_on_a_nested_attributes_collection_association_yields_only_builder
     @post.comments = [Comment.new(321), Comment.new]
     yielded_comments = []