class Trade:
    name = models.CharField(
        default='',
        max_length=50,
        blank=True,
        null=True
    )
    date = models.DateField(
        default=None,
        blank=True,
        null=True
    )



class TradeLeg(models.Model):
    trade = models.ForeignKey(
        Trade,
        on_delete=models.CASCADE
    )
    quantity = models.IntegerField(
        default=0
    )

trade = Trade.objects.filter(tradeleg__quantity = 0)