Add RSpecs and make the where condition in migration inline

db/post_migrate/20211203091642_add_index_to_projects_on_marked_for_deletion_at.rb

@@ -6,9 +6,7 @@ class AddIndexToProjectsOnMarkedForDeletionAt < Gitlab::Database::Migration[1.0]
   INDEX_NAME = 'index_projects_not_aimed_for_deletion'
 
   def up
-    marked_for_deletion_is_null = 'marked_for_deletion_at IS NULL'
-
-    add_concurrent_index :projects, :marked_for_deletion_at, where: marked_for_deletion_is_null, name: INDEX_NAME
+    add_concurrent_index :projects, :marked_for_deletion_at, where: 'marked_for_deletion_at IS NULL', name: INDEX_NAME
   end
 
   def down

spec/migrations/20211203091642_add_index_to_projects_on_marked_for_deletion_at_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!
+
+RSpec.describe AddIndexToProjectsOnMarkedForDeletionAt do
+  it 'correctly migrates up and down' do
+    reversible_migration do |migration|
+      migration.before -> {
+        expect(ActiveRecord::Base.connection.indexes('projects').map(&:name)).not_to include('index_projects_not_aimed_for_deletion')
+      }
+
+      migration.after -> {
+        expect(ActiveRecord::Base.connection.indexes('projects').map(&:name)).to include('index_projects_not_aimed_for_deletion')
+      }
+    end
+  end
+end