问题
If I have two classes:
class Group(models.Model):
name = models.CharField(...)
class Item(models.Model):
group = models.ForeignKey(Group)
published = models.DateTimeField(auto_now_add=True)
How can I make a QuerySet to select the latest published Item from each Group? I guess it should be something like
Item.objects.order_by('published').distinct('group')
but I can't make it work.
回答1:
models.py
class Group(models.Model):
name = models.CharField(max_length=100)
def __unicode__(self):
return self.name
def latest_published(self):
items = Item.objects.filter(group=self)[:1]
for item in items:
return item.published
return ''
class Item(models.Model):
group = models.ForeignKey(Group)
published = models.DateTimeField()
def __unicode__(self):
return "{0}".format(self.published)
class Meta:
ordering = ('-published',)
views.py
def myview(request):
groups = Group.objects.filter()
[.........]
template
{% for group in groups %}
{{group}} - {{group.latest_published}}<br/>
{% endfor %}
回答2:
If you don't mind loading ALL of the Item's into memory from the database, and you want to do it in a single query, then you can do this. In my scenerio, there are about 8 "Items" and about 4 "Groups" so it's barely a performance hit to do this.
from itertools import groupby
items = [list(g)[0] for k, g in groupby(Item.objects.all().order_by('group', '-published'), lambda x: x.group_id)]
Add .select_related('group') to the inner queryset if you want to be able to access the group object.
What is it doing?
- Queryset to get all items
- Grouping the items by their group
- Picking the first items from each group, and putting them into a list.
来源：https://stackoverflow.com/questions/15503535/how-can-i-select-one-object-per-each-related-object-in-django