FIX: Add unique index to prevent duplicate slugs for categories

db/migrate/20191008124357_add_unique_index_categories_on_slug.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class AddUniqueIndexCategoriesOnSlug < ActiveRecord::Migration[6.0]
+  def change
+    add_index(
+      :categories,
+      'COALESCE(parent_category_id, -1), slug',
+      name: 'unique_index_categories_on_slug',
+      unique: true
+    )
+  end
+end