begin_transaction

# File activerecord/lib/active_record/connection_adapters/abstract/transaction.rb, line 233
      def begin_transaction(isolation: nil, joinable: true, _lazy: true)
        @connection.lock.synchronize do
          run_commit_callbacks = !current_transaction.joinable?
          transaction =
            if @stack.empty?
              RealTransaction.new(
                @connection,
                isolation: isolation,
                joinable: joinable,
                run_commit_callbacks: run_commit_callbacks
              )
            else
              SavepointTransaction.new(
                @connection,
                "active_record_#{@stack.size}",
                @stack.last,
                isolation: isolation,
                joinable: joinable,
                run_commit_callbacks: run_commit_callbacks
              )
            end

          if @connection.supports_lazy_transactions? && lazy_transactions_enabled? && _lazy
            @has_unmaterialized_transactions = true
          else
            transaction.materialize!
          end
          @stack.push(transaction)
          transaction
        end
      end