# File activerecord/lib/active_record/fixtures.rb, line 480
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 { |klass_name, klass| !insert_class(@class_names, klass_name, klass) }
end

# File activerecord/lib/active_record/fixtures.rb, line 488
def [](fs_name)
  @class_names.fetch(fs_name) {
    klass = default_fixture_model(fs_name, @config).safe_constantize
    insert_class(@class_names, fs_name, klass)
  }
end