Rails: categories and sub-categories model rails

Create a model that has references to itself for a sub-category (or a sub-sub-category, etc):

class Category < ActiveRecord::Base
  has_many :subcategories, :class_name => "Category", :foreign_key => "parent_id", :dependent => :destroy
  belongs_to :parent_category, :class_name => "Category", :optional => true
end

• the has_many defines a subcategories association of the model type Category. Ie it uses the same table.
• the belongs_to defines a relation back to the parent category, accessible via @category.parent_category

For more information on model associations, has_many or belongs_to, read the Associations Basics Guide.

To create the table use this migration:

class CreateCategories < ActiveRecord::Migration
  def self.up
    create_table :category do |t|
      t.string      :text
      # table name should be in plural 
      t.references  :parent_category, foreign_key: { to_table: :categories }
      t.timestamps
    end
  end
end

Note: this table format is (slightly) different than you proposed, but I suppose that this is not a real problem.

The Migrations Guide contains more information on database migrations.

In your controller use

def index
  @category = nil
  @categories = Category.find(:all, :conditions => {:parent_id => nil } )
end

to find all categories without a parent, ie the main categories

To find all sub-categories of any given category use:

# Show subcategory
def show
  # Find the category belonging to the given id
  @category = Category.find(params[:id])
  # Grab all sub-categories
  @categories = @category.parent_category
  # We want to reuse the index renderer:
  render :action => :index
end

To add a new category use:

def new
  @category = Category.new
  @category.parent_category = Category.find(params[:id]) unless params[:id].nil?
end 

It creates a new category and sets the parent category, if it is provided (otherwise it becomes a Main Category)

Note: I used the old rails syntax (due to laziness), but for modern versions of Rails the principle is the same.

In your categories/index.html.erb you can use something like this:

<h1><%= @category.nil? ? 'Main categories' : category.text %></h1>
<table>
<% @categories.each do |category| %>
<tr>
  <td><%= link_to category.text, category_path(category) %></td>
  <td><%= link_to 'Edit', edit_category_path(category) unless category.parent.nil? %></td>
  <td><%= link_to 'Destroy', category_path(category), :confirm => 'Are you sure?', :method => :delete unless category.parent.nil? %></td>
</tr>
<% end %>
</table>
<p>
  <%= link_to 'Back', @category.parent_category.nil? ? categories_path : category_path(@category.parent_category) unless @category.nil? %>
  <%= link_to 'New (sub-category', new_category_path(@category) unless @category.nil? %>
</p>

It shows the name of the selected category (or Main Category) and all of its sub-categories (in a nice table). It links to all sub-categories, showing a similar layout, but for the sub-category. In the end it adds a 'new sub-category' link and a 'back' link.