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POWhy/how does iterating over a list and calling 'pass' each time fix this function?
 

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  1. POWhy/how does iterating over a list and calling 'pass' each time fix this function?
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    <p>I have written the following function: </p> <pre><code>def auto_update_ratings(amounts, assessment_entries_qs, lowest_rating=-1): start = 0 rating = lowest_rating ids = assessment_entries_qs.values_list('id', flat=True) for i in ids: # I have absolutely no idea why this seems to be required: pass # without this loop, the last AssessmentEntries fail to update # in the following for loop. for amount in amounts: end_mark = start + amount entries = ids[start:end_mark] a = assessment_entries_qs.filter(id__in=entries).update(rating=rating) start = end_mark rating += 1 </code></pre> <p>It does what it is supposed to do (i.e. update the relevant number of entries in <code>assessment_entries_qs</code> with each rating (starting at <code>lowest_rating</code>) as specified in <code>amounts</code>). Here is a simple example: </p> <pre><code>&gt;&gt;&gt; assessment_entries = AssessmentEntry.objects.all() &gt;&gt;&gt; print [ae.rating for ae in assessment_entries] [None, None, None, None, None, None, None, None, None, None] &gt;&gt;&gt; &gt;&gt;&gt; auto_update_ratings((2,4,3,1), assessment_entries, 1) &gt;&gt;&gt; print [ae.rating for ae in assessment_entries] [1, 1, 2, 2, 2, 2, 3, 3, 3, 4] </code></pre> <p>However, if I do not iterate through <code>ids</code> before iterating through <code>amounts</code>, the function only updates a subset of the queryset: with my current test data (approximately 250 <code>AssessmentEntries</code> in the queryset), it always results in exactly 84 <code>AssessmentEntries</code> not being updated. </p> <p>Interestingly, it is always the last iteration of the second for loop that does not result in any updates (although the rest of the code in that iteration does execute properly), <em>as well as a portion of the previous iteration</em>. The querysets are ordered_by('?') prior to being passed to this function, and the intended results are achieved if I simply add the previous 'empty' for loop, so it does not appear to be an issue with my data).</p> <p>A few more details, just in case they prove to be relevant:</p> <ul> <li><code>AssessmentEntry.rating</code> is a standard <code>IntegerField(null=True,blank=True)</code>.</li> <li>I am using this function purely for testing purposes, so I have only been executing it from iPython.</li> <li>Test database is SQLite.</li> </ul> <p><strong>Question:</strong> Can someone please explain why I appear to need to iterate through <code>ids</code>, despite not actually touching the data in any way, and why without doing so the function still (sort of) executes correctly, but always fails to update the last few items in the queryset despite apparently still iterating through them?</p>
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    1. COdisclaimer: i don't know django. try tailing the database server's log to see what SQL is executed, and when. some ORMs return generators or proxies to a SQL action. the loop might be necessary to create the objects. what happens if you add an `all()` to the first query ? i'd guess that might make the loop unnecessary by forcing an database interaction -- most ORMs want to see an all/first/one action.
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      1. USJonathan Vanasco
    2. CO@JonathanVanasco Thanks for the suggestion. Unfortunately, adding `.all()` to the first query has no apparent effect. I believe it is implicit in [values_list](https://docs.djangoproject.com/en/1.4/ref/models/querysets/#values-list).
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    3. CO@JonathanVanasco, in the Django ORM `all()` just returns a copy of the query proxy object--it does not force execution of the query it represents.
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      1. USFrancis Avila
 

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