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Ordering and 404s

5:38 with Kenneth Love

What if we get our steps out of order? What if someone puts in a bad URL?

class Meta:
    ordering = ['field1', 'field2']

This will cause the model to be ordered by field1, then field2 if there are any conflicts on field1 (two instances having the same field1 value). Finally, they'll be sorted by id if a conflict still exists.

get_object_or_404(Model, [selectors]) - Gets an object of Model by using whatever selection arguments have been given. For example: get_object_or_404(User, username='kennethlove') would try to get a User with an username set to "kennethlove". If that User didn't exist, a 404 error would be raised.

What's the long way? Consider this view:

from django.http import Http404

from .models import Course

def course_detail(request, pk):
    try:
        course = Course.objects.get(pk=pk)
    except Course.DoesNotExist:
        raise Http404()
    else:
        return render(request, 'courses/course_detail.html', {'course': course})

It's definitely more work!

If you want, you can customize your error views.

  • 0:00

    We are really tearing through this project.

  • 0:02

    Our imaginary bosses, investors, clients, whoever, would be really proud of us.

  • 0:06

    But, you know, we've introduced some functionality that we haven't addressed

  • 0:10

    yet with our order field.

  • 0:12

    And we have that 404 just waiting to happen.

  • 0:15

    So, I'm gonna open up models.py and

  • 0:19

    we have this order field here and it has this default of zero.

  • 0:23

    Now, by default,

  • 0:24

    Django orders records by their ID which is automatically generated and incremented.

  • 0:29

    The first record created gets a one.

  • 0:31

    The next one gets a two, and so on.

  • 0:33

    Now, this is awesome until you need to reorder something.

  • 0:36

    Like, as sometimes happens when I'm writing a course or

  • 0:39

    entering into our CMS here at Treehouse.

  • 0:42

    What if I forget to put in step three, and put in step four first?

  • 0:45

    Now they'll come out out of order and

  • 0:47

    you all won't know which order to watch these videos in.

  • 0:50

    That's not good.

  • 0:51

    So, we're gonna fix this, with two simple steps.

  • 0:54

    First, we're gonna add a new section to our model.

  • 0:58

    Let's call class Meta.

  • 1:00

    Django models have an optional piece to them known as class meta.

  • 1:05

    And if you did either of the peewee courses,

  • 1:08

    you've seen something similar to this already.

  • 1:10

    It's a class inside of our model class that controls how that model

  • 1:13

    does a few things.

  • 1:14

    We're gonna use it to set a default ordering for our instances.

  • 1:19

    So, we'll say ordering equals order.

  • 1:23

    So, order them by order.

  • 1:26

    And if that fails, it'll fall back to the ID.

  • 1:28

    So, this tells Django to order all of our records by their order attribute.

  • 1:32

    If there's a conflict, like two records both have an order of two.

  • 1:35

    Then, like I said, Django automatically puts those in order by their ID.

  • 1:39

    And we can tell Django to use other fields, as well.

  • 1:42

    I'll put more info about this into the teacher's notes.

  • 1:44

    So, let's go check out our course page and make sure that nothing has changed.

  • 1:49

    Nothing's changed, still looks the same, okay.

  • 1:53

    Now, let's add a new step in our admin.

  • 1:56

    So, here's Django Basics.

  • 1:58

    What's the deal with strings?

  • 2:00

    And we're going to add a new step here which is, I don't know, Using the shell.

  • 2:06

    Learn how to use Python's shell to play with the language and get help.

  • 2:17

    Now, I want this to be first.

  • 2:19

    Let's go save and continue editing.

  • 2:21

    So, we'll come right back to this.

  • 2:22

    Okay, so we have the two steps.

  • 2:25

    Now, I want this Using the Shell step to be first and it's gonna be second.

  • 2:32

    Let's check this out.

  • 2:32

    It's gonna be be second because it has the higher ID.

  • 2:36

    They both have the same order, order zero, but one has a higher ID.

  • 2:41

    So, let's fix that.

  • 2:42

    Now, we can just mark this one as a one and leave this one as a zero.

  • 2:46

    Or we can mark this one as a two and put this one as a one.

  • 2:50

    Let's do this one as a one and this one as a zero, because otherwise, the next

  • 2:55

    one I make will come in first, because it'll automatically get an order of zero.

  • 2:58

    So, we'll save and continue editing.

  • 3:02

    We come over here, now Using the Shell should be first,

  • 3:04

    What's the deal with strings should be second.

  • 3:07

    And it is, check that out.

  • 3:09

    That is awesome.

  • 3:10

    Great to know that we can control something like this so easily.

  • 3:15

    Now, though, it's time for us to address the possible 404's.

  • 3:19

    Cuz, you remember, we have this views.py and what if somebody puts in a bad URL?

  • 3:27

    So, we don't have 18 courses yet.

  • 3:33

    Look at that, I got a 404.

  • 3:35

    It's a 404, but this is actually Django's 500 error page.

  • 3:38

    Now how do I know that it's a 500 page?

  • 3:42

    Because Django gives me a handy traceback right here.

  • 3:45

    Tracebacks are only generated when there's an error with your server.

  • 3:49

    Like a Python exception.

  • 3:51

    And errors on a server should always generate a 500.

  • 3:53

    Okay, so let's fix this views so that it gives us a 404, like it should.

  • 3:59

    Instead of giving us a 500 like it just did.

  • 4:02

    So back over here in views.py, we need to import the shortcut for

  • 4:07

    throwing a 404 when an object isn't found in database.

  • 4:10

    Now, since I said it was a shortcut, you know that there's a long way of doing it.

  • 4:14

    I'll cover that in the teachers notes.

  • 4:17

    So, we're already importing render from shortcuts,

  • 4:21

    let's also import get_object_or_404.

  • 4:25

    And now, down here where we're doing our course lookup,

  • 4:29

    let's use get_object_or_404.

  • 4:33

    So I'll do it on a second line so you can compare these really quick.

  • 4:35

    Get_object_or_404, the object is of a Course

  • 4:41

    type and our look-up is where pk equals pk.

  • 4:46

    So, this part is the same, and that's the get.

  • 4:52

    And we know it's an object, because it says object, and

  • 4:55

    the object type is Course.

  • 4:58

    If that doesn't happen, we're gonna throw a 404.

  • 5:01

    So, let's delete that line, we don't need that anymore.

  • 5:03

    We're gonna save again.

  • 5:06

    Now, let's go try our bad URL.

  • 5:07

    Remember, this is a 500 error, we've got a trace back,

  • 5:12

    we've got all this other information.

  • 5:15

    And look this is a 404, we know that because it says 404 right here.

  • 5:20

    That's awesome.

  • 5:22

    Providing good errors for our users is a must.

  • 5:24

    The last thing we want is to confuse or

  • 5:26

    lose users because we didn't explain exactly what when wrong.

  • 5:29

    We can provide custom views and templates for 404, 500, and other error codes,

  • 5:34

    too if we want.

  • 5:35

    As always, I'll provide more info about that in the teacher's notes.

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