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

There is actually, via a third party app that makes it all dead
simple, it's called django-model-utils and is available here:

https://github.com/carljm/django-model-utils

Credit where it's due, I discovered this wonderful module via PyDanny
and Audrey's Two Scoops book.

Cheers
L.

On 19 April 2013 09:15, akaariai <akaariai@gmail.com> wrote:
> On 18 huhti, 20:34, deepankar bajpeyi <dbajp...@gmail.com> wrote:
>> 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
>> '''
>>
>> 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 :)
>
> Unfortunately there isn't any good way to do this, just doing an extra
> query on save seems like the best way forward.
>
> Django doesn't currently store the DB fetched values anywhere. The
> reason for this is that doing so is somewhat expensive. It would be
> nice to offer doing this as an option, this would allow for some nice
> features, like automatic update of only changed fields, and updatable
> natural primary keys.
>
> - Anssi
>
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