setup_fixtures

# File activerecord/lib/active_record/test_fixtures.rb, line 126
    def setup_fixtures(config = ActiveRecord::Base)
      if pre_loaded_fixtures && !use_transactional_tests
        raise RuntimeError, "pre_loaded_fixtures requires use_transactional_tests"
      end

      @fixture_cache = {}
      @fixture_connections = []
      @@already_loaded_fixtures ||= {}
      @connection_subscriber = nil
      @saved_pool_configs = Hash.new { |hash, key| hash[key] = {} }

      # Load fixtures once and begin transaction.
      if run_in_transaction?
        if @@already_loaded_fixtures[self.class]
          @loaded_fixtures = @@already_loaded_fixtures[self.class]
        else
          @loaded_fixtures = load_fixtures(config)
          @@already_loaded_fixtures[self.class] = @loaded_fixtures
        end

        # Begin transactions for connections already established
        @fixture_connections = enlist_fixture_connections
        @fixture_connections.each do |connection|
          connection.begin_transaction joinable: false, _lazy: false
          connection.pool.lock_thread = true if lock_threads
        end

        # When connections are established in the future, begin a transaction too
        @connection_subscriber = ActiveSupport::Notifications.subscribe("!connection.active_record") do |_, _, _, _, payload|
          connection_name = payload[:connection_name] if payload.key?(:connection_name)
          shard = payload[:shard] if payload.key?(:shard)

          if connection_name
            begin
              connection = ActiveRecord::Base.connection_handler.retrieve_connection(connection_name, shard: shard)
            rescue ConnectionNotEstablished
              connection = nil
            end

            if connection
              setup_shared_connection_pool

              if !@fixture_connections.include?(connection)
                connection.begin_transaction joinable: false, _lazy: false
                connection.pool.lock_thread = true if lock_threads
                @fixture_connections << connection
              end
            end
          end
        end

      # Load fixtures for every test.
      else
        ActiveRecord::FixtureSet.reset_cache
        @@already_loaded_fixtures[self.class] = nil
        @loaded_fixtures = load_fixtures(config)
      end

      # Instantiate fixtures for every test if requested.
      instantiate_fixtures if use_instantiated_fixtures
    end