how to get the first element and the last element using django , Location.objects.all()

this is my code.

obj_list=Location.objects.all()
first_element=obj_list[0]
last_element=obj_list[-1]

then,

return render_to_response(template_name, {
        'first_element':first_element,
        'last_element':last_element,
    })

and in the template:

{{ first_element.terminal_id}} {{last_elem开发者_StackOverflow社区ent.terminal_id}}

but it show nothing ,

what can i do ,

thanks

---

Have a look at http://docs.djangoproject.com/en/dev/topics/db/queries/#limiting-querysets

Negative indexing (i.e. Entry.objects.all()[-1]) is not supported.

Try:

first_element = Location.objects.all()[0]
last_element = Location.objects.all().reverse()[0]

-- Update 8/6/17 --

Based on @MisterRios' comment,

As of 1.6 Django supports using .first() and .last() on querysets: first_element = Location.objects.first() last_element = Location.objects.last()

Refer: https://docs.djangoproject.com/en/1.7/ref/models/querysets/#django.db.models.query.QuerySet.first

---

You might not be able to negatively index a queryset, but you can put that queryset into a list, and then index away.

locations = list(Location.objects.all())
first_element = locations[0]
last_element = locations[-1]

This is horribly inefficient though, and should only be used if there are a small number of locations in your table, and you want to keep the code simple. Otherwise, if there is a genuine need to make this efficient, see @pterk's answer, involving aggregates and Min/Max.

---

To get the last one [-1] try Location.objects.latest('id') as seen in documentation:

https://docs.djangoproject.com/en/1.3/ref/models/querysets/#latest

---

obj_list is an instance of QuerySet, and QuerySet has own methods:

obj_list.latest('pk')
obj_list.earliest('pk')
obj_list.first() 
obj_list.last()

---

Last : - Location.objects.reverse()[0]

OR

          Location.objects.all()[Location.objects.count()-1]  // BAD WAY

First: Location.objects.all()[0]

Note: Negative indexing is not supported. so, Location.objects.all()[-1] will throw you an AssertionError

---

In case anyone came here for getting first and last item of a related model - the only way of doing it efficiently are to turn related field into list or use count() to get index of last item (using Django 1.11.2):

class Parent(models.Model):
    name = models.CharField(max_length=200)


class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE, related_name='children')
    name = models.CharField(max_length=200)


class ParentTest(TestCase):
    def test_parent(self):
        # create some data
        for p in range(10):
            parent = Parent.objects.create(name=p)
            for c in range(10):
                parent.children.create(name=c)

        with self.assertRaises(AssertionError):  # can't negative index
            parents = Parent.objects.prefetch_related('children')
            for parent in parents:
                first = parent.children.all()[0]
                last = parent.children.all()[-1]

        with self.assertNumQueries(22):  # 2 for prefetch and 20 for access
            parents = Parent.objects.prefetch_related('children')
            for parent in parents:
                first = parent.children.first()
                last = parent.children.last()

        with self.assertNumQueries(22):  # 2 for prefetch and 20 for access
            parents = list(Parent.objects.prefetch_related('children'))
            for parent in parents:
                first = parent.children.first()
                last = parent.children.last()

        with self.assertNumQueries(12):  # 2 for prefetch and 10 for last
            parents = Parent.objects.prefetch_related('children')
            for parent in parents:
                first = parent.children.all()[0]
                last = parent.children.reverse()[0]

        with self.assertRaises(AssertionError):  # can't negative index
            parents = list(Parent.objects.prefetch_related('children'))
            for parent in parents:
                first = parent.children.all()[0]
                last = parent.children.all()[-1]

        with self.assertNumQueries(2):  # 2 for prefetch
            parents = Parent.objects.prefetch_related('children')
            for parent in parents:
                children = list(parent.children.all())
                first = children[0]
                last = children[-1]

        with self.assertNumQueries(2):
            parents = Parent.objects.prefetch_related('children')
            for parent in parents:
                first = parent.children.all()[0]
                last = parent.children.all()[parent.children.count() - 1]

---

it may be quite inefficient

obj_list=Location.objects.all()
first_element=obj_list[0]
last_element=obj_list[len(obj_list)-1]

---

If you have a way to sort the location objects look into Aggregates (Min and Max). http://docs.djangoproject.com/en/dev/topics/db/aggregation/

You might be tempted do Min and Max on id but try to avoid that as the order of ids is not guaranteed (at least not accross various database engines)

---

Issues query:

ERROR 2021-05-17 02:14:20,744 log 30 139680490260288 Internal Server Error: /camp/
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.7/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/opt/project/apps/web/views.py", line 419, in camp
    amount_by_daily = draw_amount_by_daily()
  File "/opt/project/apps/web/charts.py", line 16, in draw_amount_by_daily
    daily_list = DailyReport.objects.filter(category="daily_all").order_by('create_at').values("create_at")[-12:]
  File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 288, in __getitem__
    "Negative indexing is not supported."
AssertionError: Negative indexing is not supported.

daily_list = DailyReport.objects.filter(category="daily_all").order_by('create_at').values("create_at")[-12:]

How to fixed it?

daily_list = DailyReport.objects.filter(category="daily_all").order_by('-create_at').values("create_at")[:12]
result = list(reversed(daily_list))

Tips:

In [164]: l1 = [1,2,3,4,5,6]

In [165]: l1 = list(reversed(l1))

In [166]: l1
Out[166]: [6, 5, 4, 3, 2, 1]