I think many similar questions have already been asked, but I could not make up a copy-paste solution. So here is the problem:
I periodically compute analysis on my user data for reporting. I made up a model to store these computations in my sqlite database.
class report_data(models.Model):
name = models.TextField()
matrix = PickledObjectField()
creation_date = models.DateTimeField('date of creation')
I am using an append only scheme, to persist historical data, so there are many records in database, that have
- same name
- different matrix
- different creation date
My report view now needs a set, which contains that records with most recent creation date and unique names. Unfortunately sqlite does not support DISTINCT ON and the solution I found here, does not make sure, that it is always the most recent matrix, does it?
report_data.objects.all().distinct()
Does not work either, because, these records are all distinct by there key. As well as
report_data.objects.all().latest("creation_date")
does not work, because it does only return the one most recent element without taking into account the name column.
I am quite a beginner in django, in SQL I would try something like
SELECT max(date),name,matrix FROM report_data group by name
but I have not tested this.
EDIT:
By support of the comment discussion I made up some kind of solution, which looks like this:
a = report_data.objects.values('name').annotate(latest=Max('creation_date'))
Then, I got all the right creation dates, but how to get the annotated objects?
EDIT2:
Now I use a ReportManager, that looks like this:
class ReportManager(models.Manager):
def recent(self):
a = report_data.objects.values("name").annotate(latest_id=Max('id'))
a = list(v['latest_id'] for v in a)
return report_data.objects.filter(id__in=a).order_by('-creation_date')
But I am not quite happy with this solution. Thanks for posting a real solution!