LoadError in Articles#index cannot load such file -- omniauth for twitter authentication in ruby on rails -

i trying twitter authentication blog. have implemented devise in application. following http://railscasts.com/episodes/235-devise-and-omniauth-revised video. when completed steps mentioned in video , tried access application, getting cannot load such file -- omniauth error. details of error message following.

extracted source (around line #12):  9:   <div id="tabs">  10:   <ul id="tabs"> 11:     <li><div style="color:#82b548;font-size: 0.9em;">  <%= time_tag(article.created_at.in_time_zone("asia/calcutta")) %> </div></li> 12:     <li> <div style="color:#4895d1; font-size: 0.9em;">  <%= article.user.username if article.user %></div></li> 13:     <% if !article.nil? %> 14:       <li><div style="color:#ccd600; font-size: 0.9em;">  <%= article.comments.size %> comments </div></li> 15:       <li><div style="color:#ccd600; font-size: 0.9em;"> <%= link_to article.tags.collect(&:name).join(" ") %> </div></li> rails.root: f:/kuta/billi  application trace | framework trace | full trace app/models/user.rb:15:in `<class:user>' app/models/user.rb:1:in `<top (required)>' app/views/articles/index.html.erb:12:in `block in _app_views_articles_index_html_erb__265352894_31994844' app/views/articles/index.html.erb:6:in `each' app/views/articles/index.html.erb:6:in `_app_views_articles_index_html_erb__265352894_31994844' 

models/user.rb

class user < activerecord::base   attr_accessible :username, :email, :password, :password_confirmation, :remember_me   attr_accessible :title, :body   validates_confirmation_of :password   validates_presence_of :password, :on => :create   validates_presence_of :email, :on => :create   validates_presence_of :username, :on => :create   validates_uniqueness_of :email   validates_uniqueness_of :username     has_many :articles, :order => "created_at desc"     has_many :comments   # include default devise modules. others available are:   # :token_authenticatable,    # :lockable, :timeoutable , :omniauthable   devise :database_authenticatable, :registerable, :omniauthable,    :recoverable, :rememberable, :trackable, :validatable    def self.from_omniauth(auth)    where(auth.slice(:provider, :uid)).first_or_create |user|     user.provider = auth.provider     user.uid = auth.uid     user.username = auth.info.nickname     end   end    def self.new_with_session(params, session)     if session["devise.user_attritubes"]       new(session["devise.user_attributes"], without_protection: true) |user|       user.attributes = params       user.valid?       end     else      super     end   end    def password_required?    super && provider.blank?   end  end 

i think error lies in 12 line of code , regarding user. not know missing. let me know if need more code pasted.