Rails 4: passing custom data and form data in ajax call - ruby-on-rails

Question

I am trying to submit a simple form using ajax call this way
$('#save-artificial-form').click(function () {
var fd = $('#categories-form').serialize();
console.log(fd);
var url = "/categories";
$.ajax({
type: 'POST',
url: url,
data: $('#categories-form').serialize(),
success: function() {
return true;
}
});
It works perfectly fine and save form data. But I want to add one more data category_type but I don't know how to do this. I want something like this
data: $('#categories-form').serialize(), category_type: "advanced"
How can I do this? It is okay if category_type is added/pushed in the fd variable above.
code in my form
<%= form_for(#category, :html => {:id => "categories-form", class: "form-alt"}) do |f| %>
<% if #category.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(#category.errors.count, "error") %> prohibited this category from being saved:</h2>
<ul>
<% #category.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= f.label :name %><br>
<%= f.text_field :name %>
</div>
<div class="field">
<%= f.label :enabled %><br>
<%= f.check_box :enabled %>
</div>
<div style="margin: 6px 0px 0 0 !important;">
<%= link_to "This is test", 'javascript:void(0)', id: "save-artificial-form", :rel => "tooltip", :title => "save" %>
</div>
<% end %>

Answer 1

Option 1:
Use a hidden field and it will be added automatically to the serialized string
<%= f.hidden_field :category_type, value: 'advanced' %>
Option 2:
Concat the missing value to the serialized string ie. "param=value"+ "&missing_data=bar"
data: $('#categories-form').serialize() + '&category_type=advanced'