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POIterating over a large Django queryset while the data is changing elsewhere
 

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    <p>Iterating over a queryset, like so:</p> <pre><code>class Book(models.Model): # &lt;snip some other stuff&gt; activity = models.PositiveIntegerField(default=0) views = models.PositiveIntegerField(default=0) def calculate_statistics(): self.activity = book.views * 4 book.save() def cron_job_calculate_all_book_statistics(): for book in Book.objects.all(): book.calculate_statistics() </code></pre> <p>...works just fine. However, this is a cron task. <code>book.views</code> is being incremented while this is happening. If <code>book.views</code> is modified while this cronjob is running, it gets reverted.</p> <p>Now, <code>book.views</code> is not being modified by the cronjob, but it is being cached during the <code>.all()</code> queryset call. When <code>book.save()</code>, I have a feeling it is using the old <code>book.views</code> value.</p> <p>Is there a way to make sure that only the <code>activity</code> field is updated? Alternatively, let's say there are 100,000 books. This will take quite a while to run. But the <code>book.views</code> will be from when the queryset originally starts running. Is the solution to just use an <code>.iterator()</code>?</p> <p><strong>UPDATE:</strong> Here's effectively what I am doing. If you have ideas about how to make this work well inline, then I'm all for it.</p> <pre><code>def calculate_statistics(self): self.activity = self.views + self.hearts.count() * 2 # Can't do self.comments.count with a comments GenericRelation, because Comment uses # a TextField for object_pk, and that breaks the whole system. Lame. self.activity += Comment.objects.for_model(self).count() * 4 self.save() </code></pre>
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