[Python] Django join where values not null

I have 3 Tabels in Django

class Transactions(models.Model):
public_key = models.ForeignKey(Keys, on_delete=models.CASCADE)
txid = models.CharField(max_length=30)
timestamp = models.DateTimeField()
hash = models.CharField(max_length=64)
block = models.IntegerField()
amount = models.IntegerField()
saldo = models.IntegerField()
fee = models.IntegerField()
fiat_CHF = models.FloatField(default=0)
fiat_USD = models.FloatField(default=0)
fiat_EUR = models.FloatField(default=0)
position = models.IntegerField(default=0)

class Price(models.Model):
date = models.DateTimeField()
price = models.FloatField()
fiat = models.ForeignKey(Currencies, on_delete=models.DO_NOTHING)

class PriceTextBackup(models.Model):
date = models.CharField(max_length=20)
price = models.FloatField()
fiat = models.ForeignKey(Currencies, on_delete=models.DO_NOTHING)


Now i want to 'join' transactions with price on the date, if it is null i want to check the text backup, if it is still null, i want it to be 0.

Is that possible? and how?

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