FIX: The constraint on parent categories wasn't working properly.

db/migrate/20140623195618_fix_categories_constraint.rb

@@ -0,0 +1,11 @@
+class FixCategoriesConstraint < ActiveRecord::Migration
+  def change
+    remove_index :categories, name: 'index_categories_on_parent_category_id_and_name'
+
+    # Remove any previous duplicates
+    execute "DELETE FROM categories WHERE id IN (SELECT id FROM (SELECT id, row_number() over (partition BY parent_category_id, name ORDER BY id) AS rnum FROM categories) t WHERE t.rnum > 1)"
+
+    # Create a proper index for two categories not to have the same parent
+    execute "CREATE UNIQUE INDEX unique_index_categories_on_name ON categories (COALESCE(parent_category_id, -1), name)"
+  end
+end