FEATURE: create permalink when category slug is changed

app/models/category.rb

@@ -46,6 +46,9 @@ class Category < ActiveRecord::Base
 
   after_update :rename_category_definition, if: :name_changed?
 
+  after_create :delete_category_permalink
+  after_update :create_category_permalink, if: :slug_changed?
+
   after_save :publish_discourse_stylesheet
 
   has_one :category_search_data
@@ -447,6 +450,24 @@ SQL
     end
   end
 
+  def create_category_permalink
+    old_slug = changed_attributes["slug"]
+    if self.parent_category
+      Permalink.create(url: "c/#{self.parent_category.slug}/#{old_slug}", category_id: id)
+    else
+      Permalink.create(url: "c/#{old_slug}", category_id: id)
+    end
+  end
+
+  def delete_category_permalink
+    if self.parent_category
+      permalink = Permalink.find_by_url("c/#{self.parent_category.slug}/#{slug}")
+    else
+      permalink = Permalink.find_by_url("c/#{slug}")
+    end
+    permalink.destroy if permalink
+  end
+
   def publish_discourse_stylesheet
     DiscourseStylesheets.cache.clear
   end

spec/models/category_spec.rb

@@ -316,6 +316,30 @@ describe Category do
       expect { @category.update_attributes(name: 'Troutfishing', topic_id: nil) }.to_not raise_error
     end
 
+    it "creates permalink when category slug is changed" do
+      @category.update_attributes(slug: 'new-category')
+      expect(Permalink.count).to eq(1)
+    end
+
+    it "creates permalink when sub category slug is changed" do
+      sub_category = Fabricate(:category, slug: 'sub-category', parent_category_id: @category.id)
+      sub_category.update_attributes(slug: 'new-sub-category')
+      expect(Permalink.count).to eq(1)
+    end
+
+    it "deletes permalink when category slug is reused" do
+      Fabricate(:permalink, url: "/c/bikeshed-category")
+      Fabricate(:category, slug: 'bikeshed-category')
+      expect(Permalink.count).to eq(0)
+    end
+
+    it "deletes permalink when sub category slug is reused" do
+      Fabricate(:permalink, url: "/c/main-category/sub-category")
+      main_category = Fabricate(:category, slug: 'main-category')
+      Fabricate(:category, slug: 'sub-category', parent_category_id: main_category.id)
+      expect(Permalink.count).to eq(0)
+    end
+
     it "should not set its description topic to auto-close" do
       category = Fabricate(:category, name: 'Closing Topics', auto_close_hours: 1)
       expect(category.topic.auto_close_at).to be_nil