[Python] Changed the relationship of a Foreign Key | Django model

my buyer field from the Sell model had relationship with the Buyer model like this:

class Buyer(models.Model):
name = models.CharField(max_length=200, blank=True)
description = models.CharField(max_length=200, blank=True)

class Sell(models.Model):
item = models.OneToOneField(Buy, related_name='sell', on_delete=models.CASCADE)
buyer = models.ForeignKey(**Buyer**, related_name='sell', on_delete=models.CASCADE)


But now, it's set to 'User' model as you can see below:

class Sell(models.Model):
item = models.OneToOneField(Buy, related_name='sell', on_delete=models.CASCADE)
buyer = models.ForeignKey(**User**, related_name='sell', on_delete=models.CASCADE)


When I go to Django admin, it shows me all the available users from the User model but if I try to save, I got an error like this:

insert or update on table "dma_sell" violates foreign key constraint "dma_sell_buyer_id_4d165f92_fk_dma_buyer_id"
DETAIL: Key (buyer_id)=(13) is not present in table "dma_buyer".


I'm wondering I should NOT have changed this but what is the best way to correct that? I really need to relate the buyer field with User model!

Thank you!

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