How to get model objects in the form with rails check_box?

Question

How do i get checkbox values in the form from the database? I want the form to bring the existing sub category name,and when i check the checkbox to select that particular category name and not create a new one.I have tried ryan bate's railscast but was no help to me. The realationship here is Category has_many SubCategories and SubCategory belongs_to Category.Thank you.

 <%= form_for @category ,:url=>{:action =>"create"} do |f| %>
 <%=f.text_field :category_name %>
 <%= f.fields_for :sub_categories do |s| %>
 <% @category.sub_categories.each do |sub|%>
 <%=s.check_box "name",{},sub.id %>  <!--need help here-->
 <%end%> 
 <%end%>
 <%=f.submit "submit"%>
 <%end%>

Answer 1

Based on the exchange in the comments, it appears that you want to use the checkboxes to assign SubCategory objects to a Category object. If that's the case, you're association should be that a Category has_and_belongs_to_many :sub_categories. Then your form would look something like:

<%= form_for @category ,:url=>{:action =>"create"} do |f| %>
 <%=f.text_field :category_name %>
 <% SubCategories.each do |sc| %>
    <div>
      <%= check_box_tag :sub_category_ids, sub_category_id, @category.sub_categories.include?(sc), :name => 'category[sub_category_ids][]' -%>
      <%= label_tag :sub_category_ids, sc.name -%>
    </div>
  <% end -%>
<% end %>

Which will show a category form and then list all of the sub_categories that can be assigned or unassigned by checking the checkboxes.

You will also need a join table "categories_sub_categories" for this new association and logic (likely in your controller) to handle the actual assignment.

example for your category_controller.rb

def create
    @category = Category.find(params[:id])
    #use the checked sub_category_ids from the form to find and assign the sub_categories.
    assigned_sub_categories = SubCategory.find(params[:category][:sub_category_ids]) rescue []
    @category.sub_categories = assigned_sub_categories
    if @category.save
      …
    else
      …
    end
end