I've got a code like that:
# ... obj = Model.objects.get(pk=2342) if foo: obj.foo = 'bar' if bar: obj.bar = 'baz' obj.save() Is there a good way to find out if the model instance was modified in order to prevent saving it each time the code runs?
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4 Answers
The typical pattern is to do something like:
model = Model.objects.get(pk=2342) dirty = False if foo: model.foo = 'bar' dirty = True if bar: model.bar = 'baz' dirty = True if dirty: model.save() 
Comments
Just diff the 'snapshot' instance w/ current model instance field-by-field, you could get the snapshot through copy.copy(obj) or model_cls.objects.get(pk=obj.pk).
Also you could simply compare the dumped versions:
from django.core.serializers.json import Serializer dump = Serializer.serialize([obj]) ... changed = dump == Serializer.serialize([obj]) Normally, tweak your code is easiest:
obj = Model.objects.get(pk=2342) # 'obj' is better than 'model', IMO changed = False if foo: ... obj.foo = 'bar' changed = True if bar: ... obj.bar = 'baz' changed = True if changed: obj.save() 
Comments
Django performs this check internally. An sql query wont be fired unless the model has actually changed.
UPDATE
This answer is wrong. My bad. I confused this with something else.
4 Comments
Manoj Govindan
+1. Leave that check to Django unless you have very compelling reasons to do so.
okm
I don't think so. Can you provide some proof? Or have you mixed it up w/
has_changed checking of the Django form?
okm
Furthermore, this logic is incorrect because it's the DB backend which maintains the consistency, not the model instance itself.
To do this in a more abstracted way, look at the documentation for the model method from_db(), which is a hook called when data is loaded into the model instance from the database. With this method, you can cache the original value of a field as it is loaded, and then compare that to the current value in the save() method.
Example (note, I haven't actually tested this code):
class SaveIfModified(models.Model): foo = models.TextField() @classmethod def from_db(cls, db, field_names, values): instance = super().from_db(db, field_names, values) # caveat: deferred values won't appear here if "foo" in field_names: instance._foo = values[field_names.index("foo")] return instance def save(self, *args, **kwargs): if self.foo == getattr(self, "_foo", None): return super().save(*args, **kwargs) 