class ActiveRecord::FixtureSet::ClassCache

def initialize(class_names, config)
  @class_names = class_names.stringify_keys
  @config      = config
  # Remove string values that aren't constants or subclasses of AR
  @class_names.delete_if { |k,klass|
    unless klass.is_a? Class
      klass = klass.safe_constantize
      ActiveSupport::Deprecation.warn("The ability to pass in strings as a class name to `set_fixture_class` will be removed in Rails 4.2. Use the class itself instead.")
    end
    !insert_class(@class_names, k, klass)
  }
end