1 00:00:05,330 --> 00:00:09,470 Welcome back, everyone, to this lecture series, we're a first time to talk about updating models 2 00:00:09,710 --> 00:00:12,470 and then we'll talk about updating entries in those models. 3 00:00:13,590 --> 00:00:17,370 So there may come a time when need to create a new column or attribute for a model. 4 00:00:17,490 --> 00:00:23,340 And you can easily update existing models by simply adding a new model class attribute and then migrating 5 00:00:23,340 --> 00:00:24,060 those changes. 6 00:00:25,220 --> 00:00:29,750 You should note, however, that when you're adding new fields, that is attributes to the model class, 7 00:00:29,990 --> 00:00:35,060 the existing entries will need to have some sort of default value inserted, even if it's just a null 8 00:00:35,060 --> 00:00:35,480 value. 9 00:00:35,900 --> 00:00:40,880 In fact, when we attempt to run these migrations without taking care of these sort of default value 10 00:00:40,880 --> 00:00:45,860 issues that are going to occur for the existing entries, Chengdu will actually specifically request 11 00:00:45,860 --> 00:00:50,450 us to make a decision, which is actually quite nice that we don't encounter errors further down the 12 00:00:50,450 --> 00:00:50,780 line. 13 00:00:50,840 --> 00:00:55,820 So jingles actually helping you by making sure you make the correct sort of adjustments for existing 14 00:00:55,820 --> 00:00:56,270 entries. 15 00:00:57,910 --> 00:01:03,010 Now, you're typically given two options when you run those migrations and you haven't chosen a default 16 00:01:03,010 --> 00:01:03,460 value. 17 00:01:03,610 --> 00:01:09,130 You either choose a default value on the spot, right when you create that migration file, or you can 18 00:01:09,130 --> 00:01:12,940 cancel the migration file in general and then create a default value within the model. 19 00:01:13,300 --> 00:01:14,620 It's technically kind of the same thing. 20 00:01:14,620 --> 00:01:15,760 One just delays it. 21 00:01:15,760 --> 00:01:17,270 And has you fixed the model? 22 00:01:17,530 --> 00:01:19,000 The other one just does it on the spot. 23 00:01:19,360 --> 00:01:24,460 I would say it's usually more robust to have the default live in the actual model instead of deciding 24 00:01:24,460 --> 00:01:25,570 they're at the command line. 25 00:01:25,810 --> 00:01:30,160 But keep in mind, each case is different, but again, it's usually better to have some sort of default 26 00:01:30,160 --> 00:01:32,740 value actually defined within your models, not pie. 27 00:01:33,980 --> 00:01:38,990 OK, so let's explore these ideas further in our code, and we're also going to touch on the idea of 28 00:01:38,990 --> 00:01:45,410 using validators within fields which add hardcoded constraints that are going to reject non valid entries. 29 00:01:45,800 --> 00:01:47,210 Let's head back to our code editor. 30 00:01:47,690 --> 00:01:50,450 OK, so here we are back inside of models, not PI. 31 00:01:50,870 --> 00:01:56,030 And let's imagine for our patients, we'd also like to keep track of some sort of biological statistic, 32 00:01:56,040 --> 00:01:57,050 maybe their heart rate. 33 00:01:57,530 --> 00:02:01,310 So every time they come into the office, we go ahead and take their heart rate. 34 00:02:01,610 --> 00:02:06,950 That way, we can take a record in our database and understand what their heart rate was last time or 35 00:02:06,950 --> 00:02:08,090 what their current heart rate is. 36 00:02:08,539 --> 00:02:13,310 Maybe you would want this to be a list, but right now we're just going pretend it's a single integer 37 00:02:13,310 --> 00:02:16,550 that gets updated and rewritten every time there's a patient. 38 00:02:17,000 --> 00:02:20,000 So, for example, I can say heart rate. 39 00:02:21,540 --> 00:02:22,760 Is a new attribute. 40 00:02:23,690 --> 00:02:29,300 For this patient, and it's also going to be an integer, so we're going to say model integer field. 41 00:02:30,570 --> 00:02:34,530 OK, now, if I just leave it like this, there's a couple of things. 42 00:02:34,590 --> 00:02:41,340 One is I would need to decide some sort of default value for this integer, for the existing patients, 43 00:02:41,340 --> 00:02:46,220 because all the existing patients in our database technically don't have some value for heart rate, 44 00:02:46,230 --> 00:02:47,550 we haven't actually assigned that yet. 45 00:02:47,970 --> 00:02:51,750 So let's see what happens if I try to actually make migrations. 46 00:02:52,110 --> 00:02:54,690 So I'm going to save this change in models that PI. 47 00:02:54,690 --> 00:02:55,920 So I had control s there. 48 00:02:56,370 --> 00:03:06,600 And then what I'm going to do is say Python managed our pie, make migrations office hit enter and you'll 49 00:03:06,600 --> 00:03:08,090 see this warning. 50 00:03:08,100 --> 00:03:14,250 Basically, it says it's impossible to add a non knowable field heart rate to patient without specifying 51 00:03:14,250 --> 00:03:14,890 a default. 52 00:03:14,910 --> 00:03:19,740 So this already fantastic that Django is helping us out so much in such clear language, it's saying 53 00:03:20,040 --> 00:03:23,100 this is because the data needs something to populate existing rows. 54 00:03:23,100 --> 00:03:26,610 As I mentioned, the existing patients now actually have any heart rate value. 55 00:03:27,450 --> 00:03:29,610 So then it offers you two options. 56 00:03:29,610 --> 00:03:34,710 If you pass the number one, then it allows you to provide a one off default now, which is going to 57 00:03:34,710 --> 00:03:36,060 be set on all existing rows. 58 00:03:36,060 --> 00:03:38,640 Pay no value for this column or two. 59 00:03:38,670 --> 00:03:41,880 You can quit and manually define a default value in models that pay. 60 00:03:42,390 --> 00:03:48,450 As I mentioned, I would usually recommend you to choose number two here in order to actually hard code 61 00:03:48,450 --> 00:03:49,320 the default value. 62 00:03:49,470 --> 00:03:53,400 Again, every situation is different, so maybe there are going to be instances where you just want 63 00:03:53,400 --> 00:03:59,340 to provide a one off default that overwrites the existing rows with wood having no value. 64 00:03:59,580 --> 00:04:02,040 But in general, I would say go ahead and choose Option two. 65 00:04:02,550 --> 00:04:06,630 This will essentially quit this migration, and you won't have to worry about that migration file. 66 00:04:06,930 --> 00:04:13,650 And now what we can do is inside of integer field, I can set a default and you could do this for any 67 00:04:13,650 --> 00:04:19,260 field, and the default value could be something like the average heart rate of maybe 60 beats per minute. 68 00:04:19,560 --> 00:04:23,280 Now again, it totally depends on the situation of what's a reasonable default or not. 69 00:04:23,610 --> 00:04:27,810 And you can even have a no value as your default, and there's many different ways to doing that. 70 00:04:28,140 --> 00:04:35,040 But you could also say something like null is equal to true, which essentially allows us to just be 71 00:04:35,040 --> 00:04:36,570 blank or a no value. 72 00:04:37,630 --> 00:04:42,700 But I'm going to say my default value is going to be 60 beats per minute. 73 00:04:43,450 --> 00:04:48,100 Now the other thing I want to take into consideration is later on when we are accepting patients and 74 00:04:48,100 --> 00:04:54,880 I'm typing in their first name, last name and age, maybe I want to actually perform some sort of validation 75 00:04:55,180 --> 00:04:56,170 on those numbers. 76 00:04:57,920 --> 00:05:02,870 For example, I don't want someone working at the front desk to accidentally say someone has a heart 77 00:05:02,870 --> 00:05:07,280 rate of or a resting heart rate of something like zero beats per minute because that's impossible. 78 00:05:07,290 --> 00:05:09,140 I mean, that person is basically that. 79 00:05:09,200 --> 00:05:12,680 I also want to make sure that the age range is something reasonable. 80 00:05:13,040 --> 00:05:15,230 This is where validators come into play. 81 00:05:15,710 --> 00:05:23,170 So we end up saying the imports for validation can come for like this, we're going to say from jingo 82 00:05:23,600 --> 00:05:26,840 dark core dot validators. 83 00:05:27,800 --> 00:05:28,490 Imports. 84 00:05:28,520 --> 00:05:33,290 And then there's a bunch of built in validators, and you can check these out if you just Google, search 85 00:05:34,040 --> 00:05:35,160 Django validators. 86 00:05:35,180 --> 00:05:37,550 There is a documentation page on validators. 87 00:05:37,970 --> 00:05:40,610 You can technically write your own custom validators. 88 00:05:40,880 --> 00:05:46,070 But a lot of times the built in ones are more than enough for us, like a URL validator or the fact 89 00:05:46,070 --> 00:05:50,270 that there's min and max value, as well as min and maxlength validators. 90 00:05:50,600 --> 00:05:56,750 So you can see here there's a min value validator, as well as a max length validator. 91 00:05:56,900 --> 00:05:59,330 So let's see how we actually use these going to come back here. 92 00:06:00,860 --> 00:06:06,410 The first thing is we actually import those classes, so I'm going to import Max. 93 00:06:07,880 --> 00:06:15,770 Value validator, and I'm also going to import Min value validator, so from jingo without core that 94 00:06:15,770 --> 00:06:18,260 validators, you import the relevant validators. 95 00:06:18,320 --> 00:06:19,370 Let's step number one. 96 00:06:19,670 --> 00:06:23,630 And if you're wondering what validators are available to you, you simply check the documentation here. 97 00:06:24,350 --> 00:06:31,730 OK, so now that I have the validators ready, then I can go to whatever fields need some sort of validation 98 00:06:31,730 --> 00:06:32,090 check. 99 00:06:32,450 --> 00:06:34,500 And I can say the following like h. 100 00:06:34,610 --> 00:06:38,550 I can say validators is equal to. 101 00:06:39,020 --> 00:06:42,920 And then I just pass any list of the instances of those validators. 102 00:06:43,070 --> 00:06:49,580 So, for example, max value validator, if I take a look at that in the documentation, I can see that 103 00:06:49,580 --> 00:06:53,150 it's a class call and it essentially just takes in a limit value. 104 00:06:53,360 --> 00:06:58,130 You can also rewrite the message that is reported back, but typically the validation error code is 105 00:06:58,130 --> 00:06:59,600 enough and informative enough for you. 106 00:06:59,600 --> 00:07:00,380 Not need to do that. 107 00:07:00,680 --> 00:07:02,780 Instead, just really focus on what's the limiting value. 108 00:07:03,050 --> 00:07:04,280 So what's the max value? 109 00:07:04,400 --> 00:07:05,480 Someone could have for agent? 110 00:07:05,490 --> 00:07:07,460 What's a minimum value someone can have for age? 111 00:07:08,360 --> 00:07:10,550 We come back here and we can say the following. 112 00:07:10,580 --> 00:07:19,970 So my first validator is going to be a min value for essentially an instance of min value validator. 113 00:07:20,750 --> 00:07:23,270 And here I'm going to say the minimum age is zero. 114 00:07:23,450 --> 00:07:24,200 So that's my limit. 115 00:07:24,860 --> 00:07:31,100 Then I add one for max value validator, and this one may be a little trickier because it's like, what's 116 00:07:31,100 --> 00:07:33,620 the maximum age I'm allowing a patient to have? 117 00:07:34,070 --> 00:07:36,530 I'm going to say something like one hundred and twenty. 118 00:07:36,950 --> 00:07:38,390 Maybe that's too young. 119 00:07:38,390 --> 00:07:38,660 Who knows? 120 00:07:38,660 --> 00:07:44,190 In the future we're living to be 130, 150 years old, but for right now, we'll say that's OK. 121 00:07:44,240 --> 00:07:48,110 That way, if someone accidentally types in one hundred and fifty, they're going to get an error and 122 00:07:48,110 --> 00:07:50,360 saying, sorry, that's out of the validation range. 123 00:07:50,510 --> 00:07:54,260 So let's add some validators for the heart rate as well. 124 00:07:54,340 --> 00:07:55,820 So we're going to say validators. 125 00:07:56,940 --> 00:08:01,350 Equals and then we're going to add in some Min Max validators as well. 126 00:08:01,740 --> 00:08:06,030 So I'm going to grab these two validators and let's pass those into our list. 127 00:08:07,450 --> 00:08:11,960 I'm also going to say that the validators that essentially the minimum value, let's say it's I'm the 128 00:08:11,980 --> 00:08:15,130 one heart rate or one beat per minute for their heart rate. 129 00:08:15,460 --> 00:08:20,320 I'm not a doctor, so it'll be kind of up to a specialist to decide what a reasonable minimum and maximum 130 00:08:20,320 --> 00:08:20,980 values here. 131 00:08:21,310 --> 00:08:25,330 And we'll say maybe a maximum value is like three hundred or something. 132 00:08:25,360 --> 00:08:29,650 Obviously, if your heart rate is either three hundred beats per minute or one beats per minute, you 133 00:08:29,860 --> 00:08:31,180 are probably in a serious situation. 134 00:08:31,180 --> 00:08:35,140 But we're going to leave that as kind of just some sort of reality check to make sure we don't get an 135 00:08:35,140 --> 00:08:37,330 entry of like a thousand or something like that. 136 00:08:37,650 --> 00:08:39,730 Essentially, just to make sure we don't get typos later on. 137 00:08:40,090 --> 00:08:42,549 OK, so now we go ahead and save those changes. 138 00:08:42,940 --> 00:08:45,590 And remember, I have a new default value here. 139 00:08:45,610 --> 00:08:50,410 I could technically add defaults for first name, last name and age, but I think heart rate is quite 140 00:08:50,410 --> 00:08:53,580 reasonable to have a default value versus first name. 141 00:08:53,590 --> 00:08:57,400 You kind of don't want a default value there because that assumes that someone is just going to come 142 00:08:57,400 --> 00:08:59,960 in and not give you their first name as a new patient. 143 00:08:59,980 --> 00:09:04,800 So we're gonna expand this again, and let's try running that migrations command again. 144 00:09:04,810 --> 00:09:07,480 So I'm saying Python managed the pie make migrations office. 145 00:09:07,750 --> 00:09:09,910 Remember to save those changes in models, not pie. 146 00:09:09,970 --> 00:09:15,880 So adding in those validators here after we import them and pass them into that list, also just do 147 00:09:15,880 --> 00:09:18,610 a quick double check to make sure parentheses and brackets are OK. 148 00:09:18,910 --> 00:09:22,780 Note that the validators is a list of instances of validation classes. 149 00:09:23,320 --> 00:09:26,710 Then what I'm going to do is go ahead and run, make migrations. 150 00:09:27,220 --> 00:09:27,680 And no. 151 00:09:27,700 --> 00:09:29,800 Now we have a new migrations file. 152 00:09:29,830 --> 00:09:36,020 So as I mentioned underneath the migrations folder, you get these kind of list of historical migrations. 153 00:09:36,040 --> 00:09:36,760 So let's check it out. 154 00:09:36,770 --> 00:09:38,560 What happens when we do another migration? 155 00:09:39,070 --> 00:09:40,710 We can see we have dependencies. 156 00:09:40,720 --> 00:09:42,000 So what are these dependencies? 157 00:09:42,010 --> 00:09:47,260 It's saying it's dependent on the initial file, which is nice because that actually allows you to then 158 00:09:47,530 --> 00:09:51,080 in the future, roll back changes in case you want to undo this migration. 159 00:09:51,640 --> 00:09:56,050 And then you can see here it's going to add fields like it's adding a new heart rate field and it's 160 00:09:56,050 --> 00:09:57,250 also altering field. 161 00:09:57,310 --> 00:10:01,900 It's altering the existing field, and it's just altering it with some validators. 162 00:10:02,080 --> 00:10:03,820 So this is actually quite readable. 163 00:10:03,820 --> 00:10:08,560 Again, as I mentioned, we don't typically edit these migration files, but it is nice to understand 164 00:10:08,560 --> 00:10:10,030 what's going on inside of them. 165 00:10:10,570 --> 00:10:15,790 So once we're OK with that again, we typically don't check that migration file was actually going to 166 00:10:15,790 --> 00:10:16,690 run this migration. 167 00:10:16,690 --> 00:10:23,740 So we're going to say Python managed that pie migrate to enter, and then it applied those migrations. 168 00:10:24,130 --> 00:10:24,460 OK. 169 00:10:24,850 --> 00:10:28,510 So that's the very basics of updating your model. 170 00:10:28,540 --> 00:10:35,410 Again, you just add in attributes and you may need to think about what defaults are reasonable given 171 00:10:35,410 --> 00:10:36,190 what you're working with. 172 00:10:36,490 --> 00:10:38,890 Ian, we also talked about adding in validators. 173 00:10:39,190 --> 00:10:44,050 Coming up next, we're going to talk about updating existing entries in our database table. 174 00:10:44,320 --> 00:10:44,860 I'll see you there.