gistfile1.py

class BankUser (Model):
  name = CharField(...)
  login = CharField(...)
  password = PasswordField(...)

  @property
  def overdrafted_accounts(self):
    // SELECT * FROM db.bankaccount WHERE user__id=1 AND money<0
    return BankAccount.objects.filter(user=self, money__lt=0).all() 

class BankAccount (Model):
  user = ForeignKey(BankUser)
  money = IntegerField(...)
  

-----
user = User.objects.get(login='vasya') // SELECT * FROM db.bankuser WHERE login='vasya'
accounts = user.overdrafted_accounts

// accounts = [BankAccount, BankAccount, BankAccount]