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Update timestamp If any instance field has changed

Author:
craigruks
Posted:
August 5, 2011
Language:
Python
Version:
1.3
Tags:
field model override compare save method instance fields
Score:
0 (after 0 ratings)

Needed a function to check if any field in an instance has changed. If so update a datetime field to now to keep track of changes. This function is an override of the save function in the model.

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def save(self, latest_update_at):
    changed = False

    # if inserting don't add the latest update at
    if self.id is None:
        super(Match, self).save()

    # otherwise check if any fields have changed
    for field in self._meta.get_all_field_names():
        if changed:
            continue

        old_value = getattr(self.__class__._default_manager.get(id=self.id), field)
        new_value = getattr(self, field)
        if new_value != old_value:
            changed = True

    if changed:
        self.latest_update_at = datetime.now()

    super(Match, self).save()

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Comments

arthur (on August 5, 2011):
<p>I think save() of the superclass is called twice when inserting (probably not what you want).</p> <p>Also, the old object is retrieved from the database for every field. You probably want something like (untested):</p> <pre>old_object = self.__class__._default_manager.get(id=self.id) changed = any(getattr(self, field) != getattr(old_object for field in self._meta.get_all_field_names()) </pre> <p>Lastly, in most cases the auto_now property of DateTimeField should suffice.</p>

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