unique eid across marketplace

gistfile1.py

@event.listens_for(Debit, 'before_insert')
def increment_balance(mapper, connection, debit):
    eid = generate_eid('D')
    result = 1
    while result:
        result = Debit.query.filter(
            Debit.marketplace == debit.marketplace,
            Debit.eid == eid
        ).count()
    debit.eid = eid