Pass existing model Instance to update_or_create callables

[James Pulec]

Since update_or_create supports callables in the defaults dictionary, I'm wondering if it's worth considering passing the existing instance (if one is found) to those callables when resolving them.

The scenario I'm running into involves wanting to merge json data. Basically, I have a JSON field on the model that I'd like to do an update_or_create for, but in the case where there's an existing instance and I'd be performing an update, I'd like the merge the existing JSON data with the data I'm passing to my defaults.

One way I can imagine solving for this case is to pass the existing model instance to callables defined in the defaults dictionary, if an existing model instance is found.

This does feel a little gross, since you'll need to check if the passed value is None, so definitely open to other suggestion.s.

Wanted to post this here before on Trac to see if there's interest, better solutions, or any concerns.

Thanks!

[Adam Johnson]

I think the scenario you're suggesting is just a little too complex for update_or_create. It doesn't actually fetch the updated model instance . You can instead write out what you want without too much extra code using get_or_create, ensuring there's a transaction and select_for_update() to lock the object whilst you modify it:

with transaction.atomic():

    obj, created = Model.objects.select_for_update().get_or_create(key=some_key, defaults={"json_field": fresh_json})

    if not created:

        obj.json_field = merge_json(obj.json_field, fresh_json)

        obj.save()

If you look inside update_or_create you'll see it essentially does this already. If you use the pattern frequently, you can always wrap it up in your own method.

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Adam

[James Pulec]

Hey Adam,

Thanks for the clarification. I totally get that this might be a bit too complex, and maybe shouldn't wind up in update_or_create(), so I can definitely just use my own manager method. I'm a little confused by this statement, though:

"It doesn't actually fetch the updated model instance."

Isn't the return obj the model instance to be updated? From what I can tell that's exactly what's happening in the source of update_or_create() as well. Am I missing something here?

[Adam Johnson]

Yes, it does, my bad.

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Adam