Fix docs for model.save() with update_fields specified and auto_now field in model

[Sebastian Haase]

Hi,

in section "Specifying which fields to save" (https://docs.djangoproject.com/en/dev/ref/models/instances/#specifying-which-fields-to-save) it reads that `update_fields` can be specified mostly for performance benefits...

However, I found that regarding a field with "auto_now=True" there is a different outcome.

IOW, specifying `update_fields` can be used to keep the old date in respective  "auto_now=True"-field UNLESS that field is also explicitly given un `update_fields`

I hope I got this correct - at least that was the impression from my own tests with Django 2.1. Than this should be more prominently stated in the documentation.

Thanks for Django.

Sebastian

[Sebastian Haase]

There are of course stackoverflow postings -e.g. https://stackoverflow.com/questions/17474057/enforce-auto-now-when-using-saveupdate-fields - is almost 6 years old.

[Sebastian Haase]

Please advise if this is the right place to report such a doc enhancement request - anyone?

BTW - in addition I also noticed that - using deserialization - object.save() gives unexpected different results for auto_now field: 

 - if the object-pk aleady existed , the field is saved as given 

 - but if it did not exist, the given value is ignored, and the current time is used instead 

... very strange if you ask me...

Cheers,

Sebastian

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