Kaminari - WillPaginate: undefined method `per' for ActiveRecord:: Relation

If you are using kaminari and will_paginate together, you will definitely face this error. In short, kaminari and will_paginate are incompatible with each other. If you are using rails_admin (which uses kaminari for pagination) and also using will_paginate, you will need to add the following code to one of the initializers under the config directory or you can create a new file, let say with the name 'will_paginate' add the code and place it into initializers directory.

if defined?(WillPaginate)
  module WillPaginate
    module ActiveRecord
      module RelationMethods
        def per(value = nil) per_page(value) end
        def total_count() count end
      end
    end
    module CollectionMethods
      alias_method :num_pages, :total_pages
    end
  end
end

That's it!! It will work like charm.
Happy Coding.