Ordering and 404s

We are really tearing through this project. 0:00 Our imaginary bosses, investors, clients, whoever, would be really proud of us. 0:02 But, you know, we've introduced some functionality that we haven't addressed 0:06 yet with our order field. 0:10 And we have that 404 just waiting to happen. 0:12 So, I'm gonna open up models.py and 0:15 we have this order field here and it has this default of zero. 0:19 Now, by default, 0:23 Django orders records by their ID which is automatically generated and incremented. 0:24 The first record created gets a one. 0:29 The next one gets a two, and so on. 0:31 Now, this is awesome until you need to reorder something. 0:33 Like, as sometimes happens when I'm writing a course or 0:36 entering into our CMS here at Treehouse. 0:39 What if I forget to put in step three, and put in step four first? 0:42 Now they'll come out out of order and 0:45 you all won't know which order to watch these videos in. 0:47 That's not good. 0:50 So, we're gonna fix this, with two simple steps. 0:51 First, we're gonna add a new section to our model. 0:54 Let's call class Meta. 0:58 Django models have an optional piece to them known as class meta. 1:00 And if you did either of the peewee courses, 1:05 you've seen something similar to this already. 1:08 It's a class inside of our model class that controls how that model 1:10 does a few things. 1:13 We're gonna use it to set a default ordering for our instances. 1:14 So, we'll say ordering equals order. 1:19 So, order them by order. 1:23 And if that fails, it'll fall back to the ID. 1:26 So, this tells Django to order all of our records by their order attribute. 1:28 If there's a conflict, like two records both have an order of two. 1:32 Then, like I said, Django automatically puts those in order by their ID. 1:35 And we can tell Django to use other fields, as well. 1:39 I'll put more info about this into the teacher's notes. 1:42 So, let's go check out our course page and make sure that nothing has changed. 1:44 Nothing's changed, still looks the same, okay. 1:49 Now, let's add a new step in our admin. 1:53 So, here's Django Basics. 1:56 What's the deal with strings? 1:58 And we're going to add a new step here which is, I don't know, Using the shell. 2:00 Learn how to use Python's shell to play with the language and get help. 2:06 Now, I want this to be first. 2:17 Let's go save and continue editing. 2:19 So, we'll come right back to this. 2:21 Okay, so we have the two steps. 2:22 Now, I want this Using the Shell step to be first and it's gonna be second. 2:25 Let's check this out. 2:32 It's gonna be be second because it has the higher ID. 2:32 They both have the same order, order zero, but one has a higher ID. 2:36 So, let's fix that. 2:41 Now, we can just mark this one as a one and leave this one as a zero. 2:42 Or we can mark this one as a two and put this one as a one. 2:46 Let's do this one as a one and this one as a zero, because otherwise, the next 2:50 one I make will come in first, because it'll automatically get an order of zero. 2:55 So, we'll save and continue editing. 2:58 We come over here, now Using the Shell should be first, 3:02 What's the deal with strings should be second. 3:04 And it is, check that out. 3:07 That is awesome. 3:09 Great to know that we can control something like this so easily. 3:10 Now, though, it's time for us to address the possible 404's. 3:15 Cuz, you remember, we have this views.py and what if somebody puts in a bad URL? 3:19 So, we don't have 18 courses yet. 3:27 Look at that, I got a 404. 3:33 It's a 404, but this is actually Django's 500 error page. 3:35 Now how do I know that it's a 500 page? 3:38 Because Django gives me a handy traceback right here. 3:42 Tracebacks are only generated when there's an error with your server. 3:45 Like a Python exception. 3:49 And errors on a server should always generate a 500. 3:51 Okay, so let's fix this views so that it gives us a 404, like it should. 3:53 Instead of giving us a 500 like it just did. 3:59 So back over here in views.py, we need to import the shortcut for 4:02 throwing a 404 when an object isn't found in database. 4:07 Now, since I said it was a shortcut, you know that there's a long way of doing it. 4:10 I'll cover that in the teachers notes. 4:14 So, we're already importing render from shortcuts, 4:17 let's also import get_object_or_404. 4:21 And now, down here where we're doing our course lookup, 4:25 let's use get_object_or_404. 4:29 So I'll do it on a second line so you can compare these really quick. 4:33 Get_object_or_404, the object is of a Course 4:35 type and our look-up is where pk equals pk. 4:41 So, this part is the same, and that's the get. 4:46 And we know it's an object, because it says object, and 4:52 the object type is Course. 4:55 If that doesn't happen, we're gonna throw a 404. 4:58 So, let's delete that line, we don't need that anymore. 5:01 We're gonna save again. 5:03 Now, let's go try our bad URL. 5:06 Remember, this is a 500 error, we've got a trace back, 5:07 we've got all this other information. 5:12 And look this is a 404, we know that because it says 404 right here. 5:15 That's awesome. 5:20 Providing good errors for our users is a must. 5:22 The last thing we want is to confuse or 5:24 lose users because we didn't explain exactly what when wrong. 5:26 We can provide custom views and templates for 404, 500, and other error codes, 5:29 too if we want. 5:34 As always, I'll provide more info about that in the teacher's notes. 5:35