Document how to generate Pre-Change Data for existing objects on the Custom Scripts page

[hagbarddenstore]

Change Type

Addition

Area

Functionality/features

Proposed Changes

The documentation page for Custom Scripts covers how to create new objects, but doesn't cover the caveats when updating an existing object.

Suggestion

Writing Custom Scripts

....the already existing text...

To generate the correct change log data when editing an existing object, a snapshot of the object must be taken before making any changes to the object.

if obj.pk and hasattr(obj, 'snapshot'):
    obj.snapshot()

obj.property = "New Value"

obj.save()

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[candlerb]

To generate the correct change log data when editing an existing object, a snapshot of the object must be taken before making any changes to the object.

And if you're going to do this properly, make sure you include a full_clean() as well:

...
obj.property = "New Value"
obj.full_clean()      # <<<<< NOTE!
obj.save()

Otherwise, you can set obj.property to an invalid value, and it will still be saved successfully.