python - Reload django object from database

178
votes

Is it possible to refresh the state of a django object from database? I mean behavior roughly equivalent to:

new_self = self.__class__.objects.get(pk=self.pk)
for each field of the record:
    setattr(self, field, getattr(new_self, field))

UPDATE: Found a reopen/wontfix war in the tracker: http://code.djangoproject.com/ticket/901. Still don't understand why the maintainers don't like this.

4
pythondjangodjango-models
In an ordinary SQL context, this doesn't make sense. The database object can only be changed after your transaction finishes and does a commmit. Once you've done that, you'd have to wait around for the next SQL transaction to commit. Why do that? How long are you going to wait for the next transaction? - S.Lott
This seems like a needless function; it's already possible to just re-look-up the object from the database. - Stephan
i would like this as well, but it has been shut down repeatedly here - eruciform
It is not appropriate because Django model objects are proxies. If you get the same table row into two objects - x1 = X.objects.get(id=1); x2 = X.objects.get(id=1), they will test as equal but they are different objects and state is not shared. You can change both independently and save them - the last one saved determines the state of the row in the database. Therefore it is correct to reload with simple assignment - x1 = X.objects.get(id=1). Having a reload method would lead to many people wrongly inferring that x1.f = 'new value'; (x1.f == x2.f) is True. - Paul Whipp

4 Answers

301
votes

As of Django 1.8 refreshing objects is built in. Link to docs.

def test_update_result(self):
    obj = MyModel.objects.create(val=1)
    MyModel.objects.filter(pk=obj.pk).update(val=F('val') + 1)
    # At this point obj.val is still 1, but the value in the database
    # was updated to 2. The object's updated value needs to be reloaded
    # from the database.
    obj.refresh_from_db()
    self.assertEqual(obj.val, 2)
29
votes

I've found it relatively easy to reload the object from the database like so:

x = X.objects.get(id=x.id)
16
votes

In reference to @grep's comment, shouldn't it be possible to do:

# Put this on your base model (or monkey patch it onto django's Model if that's your thing)
def reload(self):
    new_self = self.__class__.objects.get(pk=self.pk)
    # You may want to clear out the old dict first or perform a selective merge
    self.__dict__.update(new_self.__dict__)

# Use it like this
bar.foo = foo
assert bar.foo.pk is None
foo.save()
foo.reload()
assert bar.foo is foo and bar.foo.pk is not None
16
votes

As @Flimm pointed out, this is a really awesome solution:

foo.refresh_from_db()

This reloads all data from the database into the object.