module Rails::Generators::Testing::Assertions

assert_no_migration "db/migrate/create_products.rb"

path relative to the configured destination:
Asserts a given migration does not exist. You need to supply an absolute path or a

def assert_no_migration(relative)
  file_name = migration_file_name(relative)
  assert_nil file_name, "Expected migration #{relative} to not exist, but found #{file_name}"
end