loans = ses.query(Loan).filter(Loan.id.in_([1, 2])).all()
loans[0].pledge_amount = Decimal('13')
s1 = ses.begin_nested()
loans[0].body_amount = Decimal('14')
s1.commit()
s2 = ses.begin_nested()
loans[1].pledge_amount = Decimal('11')
ses.rollback()
ses.commit()
ses.close()

loans = ses.query(Loan).filter(Loan.id.in_([1, 2]))
for l in loans.all():
    print(l.id, l.body_amount, l.pledge_amount)

1 6 2
2 14 13