fields_for in a has_many relationship works, but not with a has_one relationship

has_many_fields_for.rb

# app/models/user.rb
class User < ActiveRecord::Base
  has_many :profiles
  accepts_nested_attributes_for :profiles
end

# app/models/profile.rb
class Profile < ActiveRecord::Base
  belongs_to :user
end

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  def new
    @user = User.new

    @user.profiles.build

    respond_to do |format|
      format.html # new.html.erb
      format.xml  { render :xml => @user }
    end
  end
end

# app/views/users/_form.html.erb
<%= form_for(@user) do |f| %>
  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>

  <%= f.fields_for :profiles do |p| %>
    <div class="field">
      <%= p.label :status %><br />
      <%= p.text_field :status %>
    </div>

  <% end %>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

# This works just fine. Yay!

has_one_fields_for.rb

# app/models/user.rb
class User < ActiveRecord::Base
  has_one :profile
  accepts_nested_attributes_for :profile
end

# app/models/profile.rb
class Profile < ActiveRecord::Base
  belongs_to :user
end

# app/controller/users_controller.rb
def new
  @user = User.new
  @user.build_profile

  respond_to do |format|
    format.html # new.html.erb
    format.xml  { render :xml => @user }
  end
end

# app/views/users/_form.html.erb
<%= form_for(@user) do |f| %>
  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>
  <%= f.fields_for(:profile) do |p| %>
    <div class="field">
      <%= p.label :status %><br />
      <%= p.text_field :status %>
    </div>
  <% end %>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

# FAIL! Doesn't associate the user and profile.

has_one_fields_for_opposite.rb

# app/models/user.rb
class User < ActiveRecord::Base
  belongs_to :profile
  accepts_nested_attributes_for :profile
end

# app/models/profile.rb
class Profile < ActiveRecord::Base
  has_one :user
end

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  def new
    @user = User.new

    @user.build_profile

    respond_to do |format|
      format.html # new.html.erb
      format.xml  { render :xml => @user }
    end
  end
end

# app/views/users/_form.html.erb
<%= form_for(@user) do |f| %>
  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>

  <%= f.fields_for :profile do |p| %>
    <div class="field">
      <%= p.label :status %><br />
      <%= p.text_field :status %>
    </div>
  <% end %>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

# This also works, but feels backwards.