Maintain a list of modified fields on each instance of a change-logged model

[jeremystretch]

NetBox version

v3.3-beta1

Feature type

New functionality

Proposed functionality

Implement a mechanism for tracking which attributes (if any) of a pre-existing instance have been modified from their original values. For example, when retrieving a site object from the database and then modifying the instance's name, automatically flag the name field as having been modified. This can be done by defining a list of fields named _changed_fields on the instance.

Use case

Being able to infer which fields of an instance have been modified unlocks some very interesting potential.

1. It obviates the need to manually call snapshot() on each instance in a view: This can be handled automatically on the first change to a field.
2. It enables responsive updates of denormalized (cached) database fields on other models (see Cached properties in CableTermination not updated #9788).

Database changes

No response

External dependencies

No response

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[jeremystretch]

A variation of the approach described above has been implemented as part of #6347 (cached device & VM component counts) for NetBox v3.6. We can likely iterate on that work to achieve a more general purpose solution.

[peteeckel]

Incidentally I needed something just like the functionality described just yesterday. The solution I found most useful for my purposes is this:

from netbox.models import NetBoxModel


class ObjectModificationMixin:
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if not hasattr(self.__class__, "check_fields"):
            self.__class__.check_fields = (
                {field.name for field in self._meta.fields}
                - {field.name for field in NetBoxModel._meta.fields}
                - {"id"}
            )

    @property
    def changed_fields(self):
        if self.pk is None:
            return None

        saved = self.__class__.objects.get(pk=self.pk)

        return {
            field
            for field in self.check_fields
            if getattr(self, field) != getattr(saved, field)
        }

Maybe it's useful as a starting point. The main advantage is that this works for any model without changes, and just initializes the class attribute check_fields when called for a model for the first time.

This implementation returns a set instead of a list, but that's just because it made more sense in my use case.