Will_paginate gem is not installing - Rails 4

for something reason will_paginate is not installed.

i have put the line

gem 'will_paginate', '~>3.0.6'

in my gemfile,

i even ran "gem install will_paginate" in the console.

it says installed sucessfully but using the instructions to check if will_paginate is indeed install returns false nil

reference:

>> defined? WillPaginate
>> ActiveRecord::Base.respond_to? :paginate
If any of these lines return nil/false, will_paginate has not properly loaded in your app.

and i get this error when implemented

undefined method `total_pages' for #<Post::ActiveRecord_Relation:0x007fa40773bc18>