Managing Roles with Checkboxes with Rolify - ruby-on-rails

Question

I currently have a partial that is able to manage roles via radio buttons below:
<%= simple_form_for user, :url => user_path(user), :html => {:method => :put, :class => 'form-horizontal' } do |f| %>
<h3>Change Role</h3>
<%= f.input :role_ids, :collection => Role.all, :as => :radio_buttons, :label_method => lambda {|t| t.name.titleize}, :label => false, :item_wrapper_class => 'inline', checked: user.role_ids %>
<%= f.submit "Add or Remove Role", :class => "btn" %>
<% end %>
I want to change this so i can add or remove multiple roles using checkboxes.
<%= simple_form_for user, :url => user_path(user), :html => {:method => :put, :class => 'form-horizontal' } do |f| %>
<h3>Change Role</h3>
<%= f.input :role_ids, :collection => Role.all, :as => :check_boxes, :label_method => lambda {|t| t.name.titleize}, :label => false, :item_wrapper_class => 'inline', checked: user.role_ids %>
<%= f.submit "Add or Remove Role", :class => "btn" %>
<% end %>
But this seems to not work, Here is the console response when using radio buttons:
{"utf8"=>"✓", "authenticity_token"=>"MTV5MNomlkV86ynh0SVR6OW5SG3+9BCznWqcfq2xaWM=", "user"=>{"role_ids"=>"2"}, "commit"=>"Add or Remove Role", "id"=>"4"}
and when using checkboxes:
{"utf8"=>"✓", "authenticity_token"=>"MTV5MNomlkV86ynh0SVR6OW5SG3+9BCznWqcfq2xaWM=", "user"=>{"role_ids"=>["2", "3", "7", ""]}, "commit"=>"Add or Remove Role", "id"=>"2"}
So why isn't it saving to the db? why do i have an empty spot in my array at the end?