Database structure to support sub categories

app/models/category.rb

@@ -7,14 +7,14 @@ class Category < ActiveRecord::Base
   belongs_to :topic, dependent: :destroy
   if rails4?
     belongs_to :topic_only_relative_url,
-    -> { select "id, title, slug" },
-    class_name: "Topic",
-    foreign_key: "topic_id"
+                -> { select "id, title, slug" },
+                class_name: "Topic",
+                foreign_key: "topic_id"
   else
     belongs_to :topic_only_relative_url,
-    select: "id, title, slug",
-    class_name: "Topic",
-    foreign_key: "topic_id"
+                select: "id, title, slug",
+                class_name: "Topic",
+                foreign_key: "topic_id"
   end
 
   belongs_to :user
@@ -33,6 +33,7 @@ class Category < ActiveRecord::Base
   validates :user_id, presence: true
   validates :name, presence: true, uniqueness: true, length: { in: 1..50 }
   validate :uncategorized_validator
+  validate :parent_category_validator
 
   before_validation :ensure_slug
   after_save :invalidate_site_cache
@@ -43,6 +44,7 @@ class Category < ActiveRecord::Base
   after_destroy :publish_categories_list
 
   has_one :category_search_data
+  belongs_to :parent_category, class_name: 'Category'
 
   scope :latest, ->{ order('topic_count desc') }
 
@@ -188,6 +190,15 @@ SQL
     errors.add(:slug, I18n.t(:is_reserved)) if slug == SiteSetting.uncategorized_name
   end
 
+  def parent_category_validator
+    if parent_category_id
+      errors.add(:parent_category_id, "You can't link a category to itself") if parent_category_id == id
+
+      grandfather_id = Category.where(id: parent_category_id).pluck(:parent_category_id).first
+      errors.add(:parent_category_id, "You can't have more than one level of subcategory") if grandfather_id
+    end
+  end
+
   def group_names=(names)
     # this line bothers me, destroying in AR can not seem to be queued, thinking of extending it
     category_groups.destroy_all unless new_record?

db/migrate/20131023163509_add_parent_category_id_to_categories.rb

@@ -0,0 +1,6 @@
+class AddParentCategoryIdToCategories < ActiveRecord::Migration
+  def change
+    add_column :categories, :parent_category_id, :integer
+  end
+end
+

spec/models/category_spec.rb

@@ -18,7 +18,7 @@ describe Category do
   it { should have_many :topics }
   it { should have_many :category_featured_topics }
   it { should have_many :featured_topics }
-
+  it { should belong_to :parent_category}
 
   describe "resolve_permissions" do
     it "can determine read_restricted" do
@@ -314,4 +314,31 @@ describe Category do
 
     end
   end
+
+
+  describe "parent categories" do
+    let(:user) { Fabricate(:user) }
+    let(:parent_category) { Fabricate(:category, user: user) }
+
+    it "can be associated with a parent category" do
+      sub_category = Fabricate.build(:category, parent_category_id: parent_category.id, user: user)
+      sub_category.should be_valid
+      sub_category.parent_category.should == parent_category
+    end
+
+    it "cannot associate a category with itself" do
+      category = Fabricate(:category, user: user)
+      category.parent_category_id = category.id
+      category.should_not be_valid
+    end
+
+    it "cannot have a category two levels deep" do
+      sub_category = Fabricate(:category, parent_category_id: parent_category.id, user: user)
+      nested_sub_category = Fabricate.build(:category, parent_category_id: sub_category.id, user: user)
+      nested_sub_category.should_not be_valid
+
+    end
+
+  end
+
 end