Sort by attribute not in database
This may be an easy question to answer, but I just can't seem to do this. I have a model called Computer which has a function (turned into an attribute via the property builtin) which queries the local filesystem to get a modification time. ie. Computer.object.all()[0].last_modified returns a time I want to sort by this time, but unfortunately there is no sort() method on querysets and you cannot seem to order_by() on anything other than table fields. Putting the field into the database isn't an option as it is a quickly changing variable. Any help gratefully appreciated. Chris -- You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-us...@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
Re: Sort by attribute not in database
On Feb 5, 2010, at 1:02 PM, ChrisC wrote: > This may be an easy question to answer, but I just can't seem to do > this. > > I have a model called Computer which has a function (turned into an > attribute via the property builtin) which queries the local filesystem > to get a modification time. > > ie. > Computer.object.all()[0].last_modified > > returns a time > > I want to sort by this time, but unfortunately there is no sort() > method on querysets and you cannot seem to order_by() on anything > other than table fields. > > Putting the field into the database isn't an option as it is a quickly > changing variable. > > Any help gratefully appreciated. Never forget, Django is Python. One way to do what you need would be to implement the __cmp__ special method on your Computer class. Then, if you make e.g. a list out of the results of your QuerySet, you can call sort() on the list, and you're done. The downside is that you need to make a list of your QuerySet, which may require quite a bit of memory, depending on the number of objects returned. The reason you can't order_by something that's not in the database is that the order_by method is SQL and is executed in the database as part of the query. The reason you can't sort() a queryset is that it's a generator, not a list, so it only lazily instantiates your model objects. ---Peter Herndon -- You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-us...@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
Re: Sort by attribute not in database
On 5 Feb, 21:21, Peter Herndon wrote: > Never forget, Django is Python. One way to do what you need would be to > implement the __cmp__ special method on your Computer class. Then, if you > make e.g. a list out of the results of your QuerySet, you can call sort() on > the list, and you're done. > > The downside is that you need to make a list of your QuerySet, which may > require quite a bit of memory, depending on the number of objects returned. > > The reason you can't order_by something that's not in the database is that > the order_by method is SQL and is executed in the database as part of the > query. The reason you can't sort() a queryset is that it's a generator, not > a list, so it only lazily instantiates your model objects. > Just the ticket. Many thanks Peter. I was doing a list comprehension along the lines of list = [ x for x in Computer.objects.all() ] which seemed to work, but your __cmp__ method looks much nicer. -- You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-us...@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
