pre_save vs post_save

[Alireza Savand]

Hi

What is the different between pre_save signal and post_save singal.

I know post_save will emited after model instance saved, and pre_save is before the saving the instance[?]

but what it the point of pre_save when we cannot access the object data before the data changes.

I mean user try to change the model content, like body of a post in a blog. using pre_save signal would be reasonable if i can access the post's body before the changes or in other word before the actual save() happens.

If i can't access to data before save() happends, So what is the point of pre_save ?

[Karl Sutt]

Regarding pre_save signal, it is useful for doing something with the "old" instance. Consider a use case where you might want to "archive" all changes to, say, the user's profile. Every time a user makes a change to their profile, you want to save the current state of the profile to another table, or another database, and then save the new model (which overwrites the current state and becomes the new current state). This sort of mechanism is useful for versioning your model instances. The pre_save signal allows you to create a hook that does precisely that. That is just one of the use cases, but I'm sure you can think of others.

Karl Sutt

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[Alireza Savand]

Thanks for your answer Karl

Actually yes, i want to do some versioning on model instance, but the problem is that the state of the model instance in pre_save is same as at the post_save.

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[Karl Sutt]

Could you provide some code (paste it to dpaste.org for example)? Specifically, it would be useful to see your model definition and signal connectors etc.

Karl Sutt

[Alireza Savand]

Okay

here the paste

http://dpaste.de/Oa9cM/

And the Model is just Poll model from the tutorial on the django official document.

[Karl Sutt]

Thanks. So, in onPreSave, you use kwargs['instance'], assuming it's the instance before saving (which it is, not just in the "current" state). So kwargs['instance'] in both handlers have the same state. If you want to get the current state (the old state, so to speak), you should to something like

sender.objects.get(id=kwargs['instance'].id)

This would get you the current state of the that instance.

Hope this is clear and helps.

Karl Sutt

[Alireza Savand]

Thank you very much Karl, it works!

[Karl Sutt]

Awesome! No problem.

Karl Sutt

[Alireza Savand]

Okay, i found a problem in it.

One of my models doesn't use id as a default pk.

is use different primary key, called iso.

So here the traceback

http://dpaste.de/o7kSw/

It's not working with those model with different primary key :|

[Karl Sutt]

I think you can just change the "id" in 

instance = sender.objects.get(id=kwargs['instance'].id)

 

to "iso" and that should to the trick. Try it, as I haven't verified it. And let us know.

Good luck,

Karl Sutt

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[Alireza Savand]

I tried already, another error will be issued ;), and it's totally weird

pasted at 

http://dpaste.de/PEOmN/

Exception Value: Cannot resolve keyword 'id' into field

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[Alireza Savand]

Aha i solved it

sender.objects.get(id=kwargs['instance'].id)

be 

sender.objects.get(pk=kwargs['instance'].pk)

Thank you again Karl ;)

[Karl Sutt]

Ahh, great! No problem.

Karl Sutt

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