This is a script I used to repair our marketplace after corrupting it with multiple valid cards/bank accounts per account. See https://github.com/gittip/www.gittip.com/issues/773.

repair-marketplace.py

#!/usr/bin/env python
import sys
sys.path.insert(0, 'env/lib/python2.7/site-packages')  # kludge for running
                                                       # under foreman

import balanced
import gittip
from gittip import wireup

wireup.db()
wireup.billing()


participants = gittip.db.fetchall("""

    SELECT balanced_account_uri
      FROM participants
     WHERE balanced_account_uri IS NOT NULL

""")


for participant in participants:
    account = balanced.Account.find(participant['balanced_account_uri'])

    for thing_type in ('card', 'bank_account'):
        things = getattr(account, thing_type+'s').all()
        things = [thing for thing in things if thing.is_valid]
        nvalid = len(things)
        if nvalid > 1:
            things.sort(key=lambda x: x.created_at)
            print nvalid, account.uri
            print " *" + str(things[-1].created_at)
            for thing in things[:-1]:
                print " ", thing.created_at
                thing.is_valid = False
                thing.save()