Filter Latest Record In Django

Writing my first Django app that gets messages from other applications and stores reports about them. It is performing very slow due to the following logic that I hope can be impro

Solution 1:

If you're not afraid of executing a query for every app in your database you can try it this way:

def get_latest_report():
    """ Returns the latest report from each app """
    return [app.report_set.latest('date') for app in App.objects.all()]

This adds a query for every app in your database, but is really expressive and sometimes maintainability and readability are more important than performance.

Solution 2:

If you are using PostgreSQL you can use distinct and order_by in combination, giving you the latest report for each app like so

Report.objects.order_by('-date').distinct('app')

If you are using a database that does not support the DISTINCT ON clause, MySQL for example, and you do not mind changing the default ordering of the Report model, you can use prefetch_related to reduce 500+ queries to 2 (however this method will use a lot more memory as it will load every report)

class Report(models.Model):

    # Fields

    class Meta:
        ordering = ['-date']

def get_latest_report():
    latest_reports = []
    for app in App.objects.all().prefetch_related('report_set'):
        try:
            latest_reports.append(app.report_set.all()[0])
        except IndexError:
            pass
    return latest_reports