How to check for a specific field change in a model?

I have my models.py :

class Hotel(models.Model):
name = models.CharField(max_length=20)
currency = models.ForeignKey(Currency)

class Currency(models.Mode):
code = models.CharField(max_length=3)
name = models.CharField(max_length=10)


 

Whenever the currency field in hotel is changing I need to be able to do something. So I have a function like :

 

@receiver(pre_save,sender=Hotel)
def update_something(sender,**kwargs)
obj = kwargs['instance']
old_object = Hotel.objects.get(pk=obj.pk)
''' Now I can do anything here comparing the oldo object with the instance '''
Now I can do anything here comparing the oldo object with the instance
'''
The thing is I don't want to make a query for this, since then the purpose of signals becomes stupid and I become a fool.
So I should be able to do something like :
updated = kwargs['update_fields']
new_currency = updated['currency']


 

Is their a way I can find out that change for ONLY one particular field say currency ,instead of doing a query like this. I want to get the changes related to the currency foreign key and update things before I save.

Thanks 

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