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copied!<p>QuerySets and QuerySet slicing are evaluated <em>lazily</em>. Iterating ids executes the query and makes <code>ids</code> behave like a static list instead of a QuerySet. So when you loop through <code>ids</code>, it causes <code>entries</code> later on to be a fixed set of values; but if you <em>don't</em> loop through <code>ids</code>, then <code>entries</code> is just a subquery with a <code>LIMIT</code> clause added to represent the slicing you do.</p> <p>Here is what is happening in detail:</p> <pre><code>def auto_update_ratings(amounts, assessment_entries_qs, lowest_rating=-1): # assessment_entries_qs is an unevaluated QuerySet # from your calling code, it would probably generate a query like this: # SELECT * FROM assessments ORDER BY RANDOM() start = 0 rating = lowest_rating ids = assessment_entries_qs.values_list('id', flat=True) # ids is a ValueQuerySet that adds "SELECT id" # to the query that assessment_entries_qs would generate. # So ids is now something like: # SELECT id FROM assessments ORDER BY RANDOM() # we omit the loop for amount in amounts: end_mark = start + amount entries = ids[start:end_mark] # entries is now another QuerySet with a LIMIT clause added: # SELECT id FROM assessments ORDER BY RANDOM() LIMIT start,(start+end_mark) # When filter() gets a QuerySet, it adds a subquery a = assessment_entries_qs.filter(id__in=entries).update(rating=rating) # FINALLY, we now actually EXECUTE a query which is something like this: # UPDATE assessments SET rating=? WHERE id IN # (SELECT id FROM assessments ORDER BY RANDOM() LIMIT start,(start+end_mark)) start = end_mark rating += 1 </code></pre> <p>Since the subquery in <code>entries</code> is executed <em>every time you insert</em> and it has a random order, the slicing you do is meaningless! This function does not have deterministic behavior.</p> <p>However when you iterate ids you actually execute the query, so your slicing has deterministic behavior again and the code does what you expect.</p> <p>Let's see what happens when you use a loop instead:</p> <pre><code>ids = assessment_entries_qs.values_list('id', flat=True) # Iterating ids causes the query to actually be executed # This query was sent to the DB: # SELECT id FROM assessments ORDER BY RANDOM() for id in ids: pass # ids has now been "realized" and contains the *results* of the query # e.g., [5,1,2,3,4] # Iterating again (or slicing) will now return values rather than modify the query for amount in amounts: end_mark = start + amount entries = ids[start:end_mark] # because ids was executed, entries contains definite values # When filter() gets actual values, it adds a simple condition a = assessment_entries_qs.filter(id__in=entries).update(rating=rating) # The query executed is something like this: # UPDATE assessments SET rating=? WHERE id IN (5,1) # "(5,1)" will change on each iteration, but it will always be a set of # scalar values rather than a subquery. start = end_mark rating += 1 </code></pre> <p>If you ever need to eagerly evaluate a QuerySet to get all its values at a moment in time, rather than perform a do-nothing iteration just convert it to a list:</p> <pre><code> ids = list(assessment_entries_qs.values_list('id', flat=True)) </code></pre> <p>Also the Django docs go into detail about <a href="https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated" rel="nofollow">when exactly a <code>QuerySet</code> is evaluated</a>.</p>

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