[Fixed]-Getting the object that is created before this object

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def delta(self):
    try:
        my_predecessor = self.objects\
            .filter(measurement_id=self.measurement_id, pk__lt=self.pk)\
            .order_by('pk')[-1]
    except IndexError:
        # there is no previous object
        return None
    else:
        return self.time - my_predecessor.time

self.measurement_id should be synonymous to self.measurement.pk

pk means primary key. The values are normally automatically assigned from a sequence.

pk__lt=self.pk means “objects whose pk is Less Than mine”, or “created earlier than me”. More here.

I order_by('pk'), so I ensure selecting Point objects in the order they were created (comparing integer fields is always faster than comparing DateTimeField objects. You can always replace it with order_by('time') if you will have some doubts 🙂

_id is described here. I used it to build a more optimal SQL query that only selects from one table (doesn’t join the table of the Measurement model). The difference between _id and __id.

Finally, [-1] takes the last object. Django queryset slicing works very similar to python slicing.

👤Art

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