How to use update_fields in Django signals

Question

I'm trying to use an the 'update_fields' argument passed to signals in Django.

I've got something very simple for now, namely:

@receiver(pre_save, sender=models.UserAdmin)
@receiver(pre_save, sender=models.UserGroupAdmin)
def update_timestamps(sender, instance, update_fields, **kwargs):
    print(f'Update fields: {update_fields}')

update_fields shows as None no matter what is being updated, which indicates I've not understood something.

What am I missing?

It looks like you want to find out which fields have changed, but that isn't what `update_fields` is for. The `update_fields` method allows you specify which fields to save when you call `save()`. If it's `None` in your signal, then it wasn't passed to the `save()` method when it was called. — Alasdair, Apr 14 '20 at 12:37
Searching *django signal changed fields* brings up questions like [like](https://stackoverflow.com/questions/36719566/identify-the-changed-fields-in-django-post-save-signal) [these](https://stackoverflow.com/questions/1355150/when-saving-how-can-you-check-if-a-field-has-changed/28094442#28094442) that might help you. — Alasdair, Apr 14 '20 at 12:39

iklinac · Accepted Answer · 2020-04-14T12:34:37.623

1

update_fields The set of fields to update as passed to Model.save(), or None if update_fields wasn’t passed to save().

update_fields just pass up fields that you have in set in save() method for update

They are not fields that are updated in particular case ( you could set few fields in this list but maybe only one of them will actually change in database)

edited Apr 14 '20 at 12:34

answered Apr 14 '20 at 12:29

iklinac

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I can and have read the documents. I do not understand how that answers my question – HenryM Apr 14 '20 at 12:31
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Django by default updates all fields and doesn't have memory of fields that are actually updated – iklinac Apr 14 '20 at 12:37

How to use update_fields in Django signals

1 Answers1