Почему я получаю UNIQUE constraint failed: offers_offer.currencyId_id?

Question

[sortfact333]

Я фильтрую xml модель и сохраняю ее таким образом

try:
		# check model
		offer = Offer.objects.get(id = lxml_id)

		currencyId = host_ip.find('currencyId').text
		print(currencyId)
		currency = Currency.objects.get(name = currencyId)
		print(offer.currencyId)
		if not offer.currencyId == currencyId:
			offer.currencyId = currency
			save_model = True
		
		if save_model:
			offer.save()
			print('save 3')

Но получаю UNIQUE constraint failed: offers_offer.currencyId_id
Как это исправить?