How to avoid "Can't mass-assign protected attributes" error - ruby-on-rails

Question

Even though I added accepts_nested_attributes_for to my model.
it still says "Can't mass-assign protected attributes"
What else am I supposed to do in order to avoid this???
models/user.rb
class User < ActiveRecord::Base
validates_presence_of :username
validates_uniqueness_of :username
validates_length_of :username, :within => 4..10
acts_as_messageable
has_one :user_profile
accepts_nested_attributes_for :user_profile
# Include default devise modules. Others available are:
# :token_authenticatable, :encryptable, :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable, :confirmable,
:recoverable, :rememberable, :trackable, :validatable
# Setup accessible (or protected) attributes for your model
attr_accessible :email, :password, :password_confirmation, :remember_me, :username, :user_profile_attributes
def mailboxer_email(message)
email
end
# def name
# email
# end
end
models/user_profile.rb
class UserProfile < ActiveRecord::Base
belongs_to :user
accepts_nested_attributes_for :user
attr_accessible :nickname
end
views/registration/edit.html.erb
<h2>Edit <%= resource_name.to_s.humanize %></h2>
<%= form_for(resource, :as => resource_name, :url => registration_path(resource_name), :html => { :method => :put }) do |f| %>
<%= devise_error_messages! %>
<div class="field">
<%= f.label :nickname %><br />
<%= f.fields_for :nickname_attributes, #user.user_profile do |user_profile| %>
<%= user_profile.text_field :nickname %>
<% end %>
</div>
<div><%= f.label :email %><br />
<%= f.email_field :email %></div>
<div><%= f.label :password %> <i>(leave blank if you don't want to change it)</i><br />
<%= f.password_field :password %></div>
<div><%= f.label :password_confirmation %><br />
<%= f.password_field :password_confirmation %></div>
<div><%= f.label :current_password %> <i>(we need your current password to confirm your changes)</i><br />
<%= f.password_field :current_password %></div>
<%= recaptcha_tags :display => {:theme => 'red'} %>
<div><%= f.submit "Update" %></div>
<% end %>
<h3>Cancel my account</h3>
<p>Unhappy? <%= link_to "Cancel my account", registration_path(resource_name), :confirm => "Are you sure?", :method => :delete %>.</p>
<%= link_to "Back", :back %>

Answer 1

attr_accessible defines the attributes you want the user to be able to mass assign. Just make sure it has all the attributes you want in there.
To be fair, you can remove attr_accessible if you don't care about it and the error will disappear (but all your model fields will be mass assignable).

Answer 2

in edit.html.erb
wrong:
f.fields_for :nickname_attributes,
correct:
f.fields_for :user_profile_attributes,