FIX: Add migrations to fix index on category slugs

Slugs can be the empty string, but the added index didn't account for
that. This commit changes the migration, stopping it from being unique
so that it can be applied everywhere and adds another migration that
recreates the index properly.

app/models/category.rb

@@ -889,6 +889,6 @@ end
 #  index_categories_on_reviewable_by_group_id  (reviewable_by_group_id)
 #  index_categories_on_search_priority         (search_priority)
 #  index_categories_on_topic_count             (topic_count)
-#  unique_index_categories_on_name             (COALESCE(parent_category_id, '-1'::integer), name) UNIQUE
-#  unique_index_categories_on_slug             (COALESCE(parent_category_id, '-1'::integer), slug) UNIQUE
+#  unique_index_categories_on_name             ((COALESCE(parent_category_id, '-1'::integer)), name) UNIQUE
+#  unique_index_categories_on_slug             ((COALESCE(parent_category_id, '-1'::integer)), slug) UNIQUE WHERE ((slug)::text <> ''::text)
 #

db/migrate/20191008124357_add_unique_index_categories_on_slug.rb

@@ -5,8 +5,7 @@ class AddUniqueIndexCategoriesOnSlug < ActiveRecord::Migration[6.0]
     add_index(
       :categories,
       'COALESCE(parent_category_id, -1), slug',
-      name: 'unique_index_categories_on_slug',
-      unique: true
+      name: 'unique_index_categories_on_slug'
     )
   end
 end

db/migrate/20191016124059_fix_category_slugs_index.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class FixCategorySlugsIndex < ActiveRecord::Migration[6.0]
+  def change
+    remove_index(:categories, name: 'unique_index_categories_on_slug')
+
+    add_index(
+      :categories,
+      'COALESCE(parent_category_id, -1), slug',
+      name: 'unique_index_categories_on_slug',
+      where: "slug != ''",
+      unique: true
+    )
+  end
+end