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PODjango optimize queries with for loops
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copied!<p>Is it possible to optimize Django to not run so many queries on something like this</p> <pre><code>for student in Student.objects.all(): for course in student.course_set.all(): for grade in course.grade_set.filter(student=student): # do stuff </code></pre> <p>The amount of queries is students * courses * grades which can get huge.</p> <p>*edit One possibility after getting some ideas from roseman's blog.</p> <pre><code>for grade in student.grade_set.order_by('course', 'marking_period').select_related(): if grade.marking_period_id in some_report_input: # do stuff </code></pre> <p>That's just a snippet but basically I replaced the for loops with just one for loops for the last item I care about (grades) Grades has references to everything I need (student, course, marking period). It was key to use things like marking_period_id instead of grade.marking_period (which does another query). </p> <p>The trade off is code readability. I wanted to filter out grades and organize based on a criteria. This goes from trivial to convoluted.</p> <p>This is by no means a generic solution. I'm sure there are times when this won't help at all. Please comment if you know a better way.</p> <p>Another example:</p> <pre><code>for student in students: print student for department in departments: print department failed_grades = Grade.objects.filter(course__department=department,course__courseenrollment__user=student,grade__lte=70) for failed_grade in failed_grades: print grade.course print grade.grade </code></pre> <p>A student gets enrolled in a course. A course has a department.</p>

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