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Welcome everyone to this lecture where we continue setting up our overall project with some models.

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So in order to continue setting up our library, we're going to need a model to actually understand

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what a book is.

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Things like the title of the book, the author, maybe a summary or ISBN number genre.

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And then speaking of that book and the genre, we can actually connect genres as its own model.

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So we can have genre as a model, and we can also have something like language as a model.

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So we're actually going to show you how to connect genre language and book models together.

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So essentially the genre of a book will actually be a reference to a genre model instance.

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Then we'll also have author be a model in itself.

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So, for example, while a book can have an author, the actual author object would be something more

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akin to a name, maybe a date of birth, date of death, books, etc. And then we can also have a book

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instance, which is a specific physical copy of the book.

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This might be the most confusing part, so let me hone in on it right now.

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Users of our library will eventually be able to check out a book instance, and we can have multiple

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book instances of the same book.

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For example, while you could have Catcher in the Rye be a book, you can have multiple physical book

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copies of Catcher in the Rye.

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That way, more than one person ends up checking out Catcher in the Rye.

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So those would be unique books.

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That is a unique instance of a physical Catcher in the Rye novel.

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So let's head over to our Voice Code editor and continue on.

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All right, so here I am back inside our project.

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I'm going to go to Models Dot Pi and begin to create our models.

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So this basically is just going to be typing in certain models that pi file.

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Be careful not to commit any typos here.

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You can always reference our code.

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I'm going to start off by actually creating the genre model first so you can see how we later connect

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this to the book model.

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And there's different ways of connecting models together.

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We can do something like a foreign key or even something like a many, too many.

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So first off, let's create a new class for this model called genre.

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And we inherit from models that model.

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And then I'm going to say something like this is a model representing a book genre, so we'll have an

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attribute for name of the genre.

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And then we're just going to say something like Models a character field and let's give the same max

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length of, let's say 150 characters.

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Okay.

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And then if you actually want to print out the genre, let's give it a string representation.

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So we'll say string peasant self here.

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And this will simply be something like return self taught name.

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Okay.

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So all we're doing here is setting up our class, genre and names equal to models, not character field.

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And right now you'll notice a lot of this text is white.

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So let me just quickly choose a different color scheme so we don't get confused.

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All right.

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There we go.

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Just switch the the color scheme that we hopefully a couple of things are different colors.

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So it's clear what's actually happening as far as what's a class call, what's a function call, what's

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a method, etc..

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So we have our genre class.

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Now let's set up our book model.

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And be careful again if you're indentation here.

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So the book model is essentially defining the single book title.

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You can think of it that way.

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Obviously, title is actually an attribute of a book, but this is not the book instance that we'll

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talk about later.

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So here we're going to say.

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Class book.

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Models that model.

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And here we're going to say title is equal to models and we'll have a character field for the actual

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book title.

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So I'm going to say like Maxlength is equal to take 200 characters doesn't really matter too much for

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our simple examples here next we want to do is have an author.

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Now I didn't mention later on we'll actually be creating an author model.

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So let's reference it here as a foreign key and then will actually build out that particular model so

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we can get the full reference.

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So essentially what I'm going to do is say that you have an author here, but I actually want book and

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author to be their own models.

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So this author is not just a simple character field.

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Instead, it's actually going to be linked to a specific author.

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So we're doing this story for key.

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Why do this?

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Well, an author could write multiple books, so be nice to have our own little database table of authors

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and then we can link them to the books that they have written that we later on, if we wanted to add

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more functionality, I could look up, Hey, this author and all the books they've written, sort of

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the same models, not for anarchy.

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And then we're going to say author.

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And then we can say a method on delete.

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Is equal to models that set null.

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And basically what this means is that you have to decide at a certain point when you start linking models.

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What happens if I delete this book?

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Would you also delete the author or would you delete the book if you happened to delete the author one

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way or the other?

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So there's different ways of doing this.

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And one way is to set models to set now.

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And essentially what happens here is that actually allows a database to store a null value if no author

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is selected.

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And that happens to mean that if you collect and delete two equal models that said no, that's actually

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a set the value of the books offer field to NULL if the associated author record is deleted.

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This is just a complicated way of saying that if for some reason your book exists but somebody x only

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deletes the author.

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It's not like the book suddenly disappears as well.

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You just are going to say, Hey, if you happen to odd delete the author, you will set it as null for

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this book.

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So it's kind of like the English translation of this code.

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So again, for the author on deletion of the author for this particular model, you'll set the author

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as null for the book.

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So hopefully that clarifies it.

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There's also another default behavior, which is Cascade, which means that if the author is deleted,

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the book would be deleted too.

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And sometimes maybe depending on the situation, you want that.

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So obviously there's something going on if you want to delete this author and suddenly delete all the

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books along with them because maybe it was a mistake.

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Then you may want to actually cascade that down so that actually end up being a delete is equal to models

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that cascade.

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But setting all is probably more appropriate for what we're doing here.

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We'll continue to build out book, but since I didn't mention that we're connecting this to an author

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model, let's quickly jump to the author model.

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So keep in mind more to come here for book, but let's set up this author model.

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Or C class author.

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Models not model.

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And then let's give them a first name.

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So models the character field.

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Let's just say 200 for the max length.

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Let's give them a last name as equal to models.

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The character field 200.

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And then, let's say date of birth.

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Date of birth is equal to models, and this probably makes more sense to be an actual date field.

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So have that be a date field and we'll go ahead and say NULL.

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It can be true if you don't happen to know them and will even let them leave it blank if we want to

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say blank is equal to true as well.

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Okay.

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So you could keep adding more information from on to like date of death, etc..

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Something we haven't seen before is adding a little bit of class here that can help us dictate behavior

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inside the admin view.

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Like what you want these sorted by or ordered by.

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So I can say ordering is equal to.

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And then you can have a list of how you want these ordered when you actually visit this or request this

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back.

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So I can say I want these ordered by last name.

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Karma.

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First name.

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Okay.

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And then finally, we're going to create some method calls here to actually define some URLs.

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So essentially it would return the URL to access a particular author instance.

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So the way this works is you create a method here called get absolute URL and you'll notice it actually

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autofill for me.

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But it's essentially going to be a reverse call.

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And instead of model detail, I switch this to say author detail.

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So I'm just switching for the actual name of the model.

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And then the keyword arguments is going to be here.

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You could say peak.

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Peak is equal to self peak.

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And I should point out that this does require a reverse call, which means up here I need to import

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that an essay from Django thought URLs imports reverse and we'll see that more later.

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But it's essentially they're saying, where's the URL that I could actually grab this author's detail

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from?

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So we have our author class.

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Last thing I want to actually mention here is how to get the string representation.

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So say str here passing self and we'll just return.

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You can use f string literal formatting.

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I'm going to return the last name.

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First name so I'll say last name.

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This should be self last name.

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So if last name.

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Karma.

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Self first name.

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And put that in.

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Okay.

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So that just returns, you know, something like last name, comma, first name, if you ask to actually

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print out that author.

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So, so far, so good for the author.

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You could keep adding more features if you wanted to.

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But let's head back now to book.

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Here.

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And now that we've set up author, we can see what's actually happening here.

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So if I delete this, Arthur, this book survives.

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It just sets the author to know.

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Now let's continue.

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What else do we want for the book?

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Maybe you want something like a summary.

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So, like a it's a description of the book.

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So let's go ahead and say summary of the book.

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Equals models.

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This probably didn't make sense as a character field because it's longer.

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So we'll say something like, What's a text field?

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And let's give it a longer maxlength like let's say that 600 or something with 600 characters, maybe

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that's too little, but not a big deal.

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And then if you want, you can also set this to be nulls equal to true or whatever, but we'll leave

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all the defaults there.

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So we have a title and author, a possible summary.

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Then let's go ahead and give this an ISBN number.

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So if you're checking out a book for the library, you know what I'm talking about here.

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It's not just Google Search, one ISBN number, essentially unique identifier for this book.

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So say character field and this is just going to be.

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ESPN.

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And let's give this a.

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Max length of I believe officially it's only 13 characters long and this one does have to be unique.

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We can't have multiple books of the same ISBN and if you're not familiar Price mean it's essentially

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a unique code to identify the books in the world.

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If you've heard of the Dewey Decimal System.

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All right.

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The last thing I want to do is actually connect this book to a genre.

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But unlike our simple model, where a book only has one unique author, technically there are cases

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in the real world where a book could have multiple authors.

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But in this case, we're just considering a book having one single unique author for genre.

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Let's show you how to connect multiple genres to a book.

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Maybe a book is both mystery and sci fi genre.

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So the way we do that is we actually classify this as a many to many connection, essentially saying

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that many books could have many genres.

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So for example, again, a book could be both a sci fi and a mystery novel.

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So let's go ahead and set that up.

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What I'm going to do here is set this up as a many, many fields.

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So we'll come here and say genre.

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Attribute is equal to models not and they'll say many to many field and here we connected with the genre

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class we created earlier and that's how you can connect it so that a book can then have essentially

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a list of potential genres.

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Let's also give a string representation for our book.

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I'll say def.

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That's tr self.

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And let's say it's going to return Self's title.

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It's by the simplest way of doing this.

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And let's also have it get absolute URL.

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It's going to return the URL to access a record for this book.

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So we'll say def get absolute URL.

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Self.

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And again, it's sort of model detail.

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I will say this is book detail.

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Notice I'm just using tab here.

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It's auto complete.

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So this is book detail and then we can have the identity be that.

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All right, so that's looking good.

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So next we have the actual book instance.

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So I have a book, a genre and then an author.

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And that's what I'm going to do is set up a book instance model.

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Remember, this one is actually going to be unique.

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So in that case, what I'm going to do and I'll do it here for now, I put this at the top is have you

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you ID.

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Which is a unique identifier generator.

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So essentially it will just generate a unique ID.

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You'll see why we do this in just a set.

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So we're going to say class.

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Book instance.

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Instance.

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L.S. models that model.

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And then from here, we're actually going to specify an ID attribute that is model's dot u u IED field.

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Okay.

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So essentially what's happening here is the book instance represents a specific copy of a book that

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someone might borrow and includes information maybe about whether the copies available or what date

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00:13:37,370 --> 00:13:38,650
it's actually expected back.

241
00:13:38,660 --> 00:13:42,410
So we should know, hey, this particular book is actually checked out for the library.

242
00:13:42,890 --> 00:13:44,970
So some of the fields we're actually going to be familiar with.

243
00:13:44,990 --> 00:13:49,400
One of them, however, needs to be a representation of a specific copy.

244
00:13:49,700 --> 00:13:53,480
So for that, we can just essentially borrow this from the UUID Field Library.

245
00:13:54,020 --> 00:13:56,480
So we'll say this is a primary key is true.

246
00:13:57,870 --> 00:14:04,660
And then we're going to say default is equal to and this is what we use that import you, you ID, you

247
00:14:04,770 --> 00:14:05,500
id4.

248
00:14:06,420 --> 00:14:06,780
Okay.

249
00:14:07,870 --> 00:14:08,070
Oops.

250
00:14:08,200 --> 00:14:10,440
Make sure you do it.

251
00:14:10,870 --> 00:14:11,350
There we go.

252
00:14:12,300 --> 00:14:13,350
Since we imported that.

253
00:14:14,540 --> 00:14:16,640
And then we'll connect this to a book.

254
00:14:16,700 --> 00:14:19,940
So say book is equal to models for key.

255
00:14:20,240 --> 00:14:25,580
Kind of makes sense that a book instance should have a unique foreign key relationship to a single book.

256
00:14:27,080 --> 00:14:29,140
And then we're going to say on delete.

257
00:14:30,210 --> 00:14:34,020
So what happens when you actually do a deletion of a book?

258
00:14:34,590 --> 00:14:38,040
In this case, maybe you want to do something like a restrict call.

259
00:14:39,840 --> 00:14:43,230
This essentially restricts you from deleting a book if you still have a book instead.

260
00:14:43,250 --> 00:14:46,980
So you have to delete all the book instances first before you're allowed to delete that book.

261
00:14:47,010 --> 00:14:53,430
So that means I'm going to say all delete is equal to model's dot restrict, essentially saying, hey,

262
00:14:53,730 --> 00:14:59,220
if you have a book in the library or checked out and you try to just delete Catcher in the Rye, you

263
00:14:59,490 --> 00:15:02,070
have to actually take care of all those book instances first.

264
00:15:02,070 --> 00:15:05,780
You have to pull those out first before you can do a general book from your database.

265
00:15:05,790 --> 00:15:11,160
So we're going to restrict that, and then I'm going to say, no, all is equal to true here.

266
00:15:12,910 --> 00:15:14,710
And then I think that should be enough.

267
00:15:15,100 --> 00:15:17,350
Maybe we want something like an imprint on the book.

268
00:15:18,600 --> 00:15:19,680
Let's say imprints.

269
00:15:20,850 --> 00:15:24,000
Attribute is equal to models, not character field.

270
00:15:25,210 --> 00:15:26,350
Let's give it a max length.

271
00:15:26,740 --> 00:15:30,070
Let's say 200 such essentially can imprint something out of the book.

272
00:15:30,490 --> 00:15:32,470
And then more importantly, what does it do back?

273
00:15:34,140 --> 00:15:37,350
So this makes sense for it to be a field or say, the field.

274
00:15:38,650 --> 00:15:39,720
No is equal to true.

275
00:15:39,730 --> 00:15:41,620
So if it's null, that means it's in the library.

276
00:15:42,400 --> 00:15:44,980
And also, let it be blank if we just want to leave it blank.

277
00:15:45,920 --> 00:15:46,280
Okay.

278
00:15:47,060 --> 00:15:51,670
So we can have an attribute actually be a selection of something.

279
00:15:51,680 --> 00:15:56,480
So you can say, hey, what's the actual loan status for a particular book instance?

280
00:15:56,630 --> 00:15:57,500
How would we do that?

281
00:15:57,920 --> 00:16:01,580
Well, I can create an attribute that is essentially a tuple here.

282
00:16:01,970 --> 00:16:08,870
So for example, I can say loan status is equal to a couple of tuples where I'm going to have a parent

283
00:16:08,870 --> 00:16:09,170
here.

284
00:16:09,470 --> 00:16:11,420
We're going to say M for maintenance.

285
00:16:15,160 --> 00:16:17,680
Then we're going to say, Oh.

286
00:16:18,560 --> 00:16:19,280
For.

287
00:16:20,790 --> 00:16:21,480
On loan.

288
00:16:23,670 --> 00:16:25,620
We're going to say if it's available.

289
00:16:28,920 --> 00:16:30,690
And then I'm going to say are.

290
00:16:32,120 --> 00:16:33,740
If it's reserved.

291
00:16:36,290 --> 00:16:41,600
And then finally, let's now that we have this lone Sarasate tribute, we can use that to actually create

292
00:16:41,720 --> 00:16:43,370
a general status, that tribute.

293
00:16:45,250 --> 00:16:50,230
So say this is the model's character field and this will show you how to construct a field based off

294
00:16:50,230 --> 00:16:51,040
other attributes.

295
00:16:51,460 --> 00:16:57,640
So, for example, the max length of this field is just going to be one y1 because we're basing it off

296
00:16:57,640 --> 00:16:59,040
m, o or r.

297
00:16:59,620 --> 00:17:01,300
So the max length is one.

298
00:17:02,750 --> 00:17:03,710
The choices.

299
00:17:04,280 --> 00:17:04,910
Sorry about that.

300
00:17:05,390 --> 00:17:06,380
The choices here.

301
00:17:07,300 --> 00:17:10,210
Are going to be your loan status, are your choices.

302
00:17:11,500 --> 00:17:13,119
It can be blank if need be.

303
00:17:14,930 --> 00:17:17,930
The default behavior will be maintenance.

304
00:17:19,190 --> 00:17:20,450
So say default is M.

305
00:17:21,910 --> 00:17:23,290
And that is your status.

306
00:17:25,150 --> 00:17:26,950
Let's put that all in one line there so you can see it.

307
00:17:27,700 --> 00:17:28,210
There we go.

308
00:17:28,600 --> 00:17:30,290
So that's the status of the book.

309
00:17:30,310 --> 00:17:33,610
And then finally, if you want, we can set up a meta class for the ordering.

310
00:17:34,820 --> 00:17:38,480
So Saint Mata ordering here is when it's due back.

311
00:17:41,080 --> 00:17:41,980
What's the mixture?

312
00:17:41,980 --> 00:17:43,050
This actually matches here?

313
00:17:43,270 --> 00:17:43,780
Do back.

314
00:17:43,960 --> 00:17:44,650
Okay, perfect.

315
00:17:45,280 --> 00:17:51,550
And let's actually create an instance representation of how to actually print out a book here self.

316
00:17:52,960 --> 00:17:54,130
And then I was going to return.

317
00:17:54,190 --> 00:17:54,940
Let's see if.

318
00:17:56,260 --> 00:17:56,770
Died.

319
00:17:58,130 --> 00:18:02,780
And then it's probably also applicable to print out the actual book title itself.

320
00:18:02,790 --> 00:18:04,260
So it's that title.

321
00:18:04,640 --> 00:18:09,290
So I can't just say self that title because technically book instances don't have titles.

322
00:18:09,500 --> 00:18:15,020
However, book instances do have a book and the book is a foreign key, which in turn actually has the

323
00:18:15,020 --> 00:18:15,410
title.

324
00:18:15,440 --> 00:18:19,640
So the way I would reference that is I would say soft dot and then I would say self.

325
00:18:19,640 --> 00:18:21,800
That book, the self, that title.

326
00:18:24,540 --> 00:18:24,900
Okay.

327
00:18:24,930 --> 00:18:28,710
And that's how you can actually grab the title based off a foreign key relationship.

328
00:18:28,740 --> 00:18:32,040
So again, it's going to say this book instance is linked to a book.

329
00:18:32,340 --> 00:18:36,330
Then grab that book's title in order to print it out.

330
00:18:36,360 --> 00:18:40,230
If you ask for a book instance because we don't just want the idea, it's just going to be a random

331
00:18:40,230 --> 00:18:41,220
number that we won't recognize.

332
00:18:41,220 --> 00:18:43,560
It'd be nice to actually see the book title there just in case.

333
00:18:44,280 --> 00:18:48,660
Now, I did mention maybe you want a model for something like language.

334
00:18:48,690 --> 00:18:53,850
So let's imagine we're starting to expand the library and somebody donates a book in Spanish and now

335
00:18:53,850 --> 00:18:57,240
we have the same books, but now they're actually in different languages.

336
00:18:57,270 --> 00:18:58,140
So how do we do that?

337
00:18:58,530 --> 00:19:03,480
It probably makes sense to actually set up a language class and it would kind of operate similar to

338
00:19:03,480 --> 00:19:05,670
genre, but we would later on add it to a book.

339
00:19:05,820 --> 00:19:06,780
So let me show you.

340
00:19:07,830 --> 00:19:08,790
What this would look like.

341
00:19:09,000 --> 00:19:10,080
So I'll say class.

342
00:19:11,290 --> 00:19:14,230
And I'm going to now make a new class called My Language Class.

343
00:19:15,590 --> 00:19:16,550
Models that model.

344
00:19:17,810 --> 00:19:18,770
And it's gonna be pretty simple.

345
00:19:18,770 --> 00:19:19,490
She's going to be a name.

346
00:19:20,710 --> 00:19:22,960
So the name of the language should be a character field.

347
00:19:23,140 --> 00:19:25,810
Let's say Maxlength is equal to 200.

348
00:19:25,810 --> 00:19:33,100
So this is something I would say like Spanish, French, Japanese, etc. and then can be a simple string

349
00:19:33,100 --> 00:19:37,240
representation here to just return self thought the name of the language.

350
00:19:37,690 --> 00:19:42,550
Okay, the question becomes what's the best way to connect this language to our present book model?

351
00:19:42,760 --> 00:19:46,910
Well, in this case, a book is probably just in a single language.

352
00:19:46,930 --> 00:19:51,910
It wouldn't make sense to make it exactly like genre, because typically when you're thinking of the

353
00:19:51,910 --> 00:19:55,360
language of a book, it's not going to be both in English and Japanese.

354
00:19:55,420 --> 00:19:59,770
Now, maybe there's actually books on learning a language, which would be tricky, but in that case,

355
00:19:59,770 --> 00:20:02,330
you'd probably want to have it in the learner's language.

356
00:20:02,350 --> 00:20:07,300
So that is to say, if you have a book that teaches an English speaker how to speak Japanese, the language

357
00:20:07,300 --> 00:20:10,780
of that book should probably just be English, not both English and Japanese.

358
00:20:11,200 --> 00:20:14,350
So in that case, we would just come here and make it a foreign key.

359
00:20:15,220 --> 00:20:17,440
So I would say language and helps.

360
00:20:17,530 --> 00:20:18,490
That should be lowercase.

361
00:20:19,300 --> 00:20:23,290
The language of the book is equal to models that foreign key.

362
00:20:24,400 --> 00:20:26,440
And then I would think it up to language.

363
00:20:27,880 --> 00:20:28,390
There we go.

364
00:20:28,840 --> 00:20:32,680
So I would have each book now having some sort of language.

365
00:20:32,980 --> 00:20:36,010
Now the question is, what do we do if we were to delete a language?

366
00:20:36,400 --> 00:20:43,510
So oddly, plie makes more sense to just set this as the also, I'll say models that set null and we'll

367
00:20:43,510 --> 00:20:43,990
say null.

368
00:20:43,990 --> 00:20:44,770
That can be true.

369
00:20:45,820 --> 00:20:46,120
Okay.

370
00:20:46,390 --> 00:20:49,300
So that's how you could add in a new field for the book.

371
00:20:49,840 --> 00:20:53,980
So save all of that and let's go ahead and run some migrations.

372
00:20:54,010 --> 00:20:57,040
Remember, we have to first make the migrations and then we need to actually migrate.

373
00:20:57,580 --> 00:20:59,890
So first off, let's check here.

374
00:20:59,940 --> 00:21:03,580
Looks like we have some hints telling us we maybe have something up.

375
00:21:04,030 --> 00:21:07,990
So in the author last name, character fields must define a maxlength attribute.

376
00:21:08,350 --> 00:21:13,370
So let me come down here to author, and it looks like you just typed in 200 instead of the maxlength.

377
00:21:13,390 --> 00:21:14,170
I'll fix that now.

378
00:21:14,920 --> 00:21:19,150
So Maxlength is equal to 200 and we'll set that up first name there.

379
00:21:20,050 --> 00:21:26,380
And so then on book author it says The field specifies Anderlecht is equal to set nul but cannot be

380
00:21:26,380 --> 00:21:30,390
null set null equals true argument on the field or change the oblique rule.

381
00:21:30,400 --> 00:21:32,710
So we'll come back to author or excuse me, book author.

382
00:21:33,070 --> 00:21:34,510
Here it says set null.

383
00:21:34,900 --> 00:21:37,480
Which means I should say null can be true.

384
00:21:38,830 --> 00:21:39,320
There we go.

385
00:21:39,340 --> 00:21:40,750
So now we'll go ahead and save that.

386
00:21:41,350 --> 00:21:43,360
And it looks like there was no complaints.

387
00:21:43,360 --> 00:21:47,290
So did those checks for us, which is really cool that it's actually doing those checks even just on

388
00:21:47,290 --> 00:21:47,710
saving.

389
00:21:48,220 --> 00:21:52,390
So what I'm going to do now is I should be ready to actually run the migrations.

390
00:21:52,390 --> 00:22:01,960
So let me do controls, see here and say up here I'm going to say Python managed that py make migrations.

391
00:22:04,280 --> 00:22:04,700
Enter.

392
00:22:04,970 --> 00:22:05,960
Looks like it's all working though.

393
00:22:05,960 --> 00:22:07,820
I'll say python manage that.

394
00:22:07,820 --> 00:22:08,600
Pi migrate.

395
00:22:08,780 --> 00:22:12,230
Hopefully we didn't permit any typos there, but it looks like everything worked.

396
00:22:12,440 --> 00:22:14,060
Okay, so we talked a lot of code.

397
00:22:14,390 --> 00:22:16,010
If you're starting to get errors here, definitely.

398
00:22:16,010 --> 00:22:19,610
It's kind of thing where you probably want to just reference our provided code.

399
00:22:20,060 --> 00:22:20,390
Okay.

400
00:22:20,900 --> 00:22:21,320
Thanks.

401
00:22:21,320 --> 00:22:22,760
And I will see you at the next lecture.