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All right.
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So in the last lesson, we looked at how you can install, setup, and configure Realm to be our method of
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persisting data.
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Now, in this lesson, I'm going to delete all of this so that we don't keep adding the same piece of data
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into our Realm.
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And I'm also going to delete this try realm.write.
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Now, I'm going to leave this do-catch block where we initialize our brand-new Realm. And let's not worry too
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much about these warnings for now.
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So I'm going to go ahead and delete this Data class that we created that we use as just as an example,
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and I'm going to replace this Core Data data model with a class called Category and a class called Item.
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So let's go into our Data Model folder, hit command Nto create a new Swift file
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and this is going to be called Item and we're going to hit Create,
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and then I'm going to create another one called Category and hit Create.
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So now we have both of our entities, or in this case, classes and we're going to use them to represent
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all the properties and relationships that we've specified inside Core Data.
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So inside Item, I first have to import Realm Swift in order to use it, and then I'm gonna create a new
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class called Item and the superclass is going to be a Realm Object. And it's going to have two properties.
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one which is the title, which is going to be a String, that's going to start off as empty.
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And the other one is going to be the property done which is going to be a Boolean and this is going
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to start off as false.
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And, remember, in order to create properties using Realm, we have to add the @objc modifier and
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also the dynamic keyword.
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So let's go ahead and add that to both of our properties.
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And now this is a complete representation of our item entity using properties instead of attributes.
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Now,  you'll see that it gives you an error saying that this is an invalid redeclaration of Item.
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So it's saying that there's already a class called Item,
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but where where is it?
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I don't see another class called Item.
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What's going on here?
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Well, do you remember that we said when you create a new data model using Core Data,
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If you select the entity and you select Coadegen as Class Definition, then it's going to create a class
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called Item with these properties, or much the equivalent to that?
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So you could either select Codegen Manual/None in order to not generate those Class Definitions, or in
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our case, we're actually just going to delete our Data Model because not going to be the one that we're
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going to use from now on.
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So I'm going to right click and hit Delete.
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So, now it no longer exists,
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and if we hit command B to build and, essentially, refresh, then that will get rid of that error where it
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says that this item class is a redeclaration.
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Now, we'll end up with more errors because, remember, for most of our view controllers, we're using our
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Core Data NSManagedObjects, and now we're switching it to Realm objects,
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so there's going to be a little bit of fixing in there.
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But let's first finish creating our Data Models.
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So we've done that for item, and now we have to move on to category.
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And I'm actually going to just change the order here because it's Category then Items, and that makes
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a little bit more sense to my brain.
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So as a challenge, I'd like you to set up your Category data model to include a property called name which
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is going to be of data type string and it's going to start off with the value of an empty string.
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So go ahead and pause the video and give it a go.
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All right.
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So that is pretty simple task.
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All we have to do in order to use Realm is, of course, import Realm Swift, and then we're going to create
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a new class that's called Category and it's going to inherit the Object class, and the code in here is
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going to be an Objective-C modifier and the dynamic keyword,
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and then we're going to say var name,
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name of the category, which is going to be a string,
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and it's going to start off being an empty string.
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All right.
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So that's it.
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That was done.
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Did you manage to get that?
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Now, in addition to having these properties, if you remember, our Data model using Core Data also had relationships
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where we could specify that categories point towards items and items point back to categories.
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So let's go ahead and see how we can do that using Realm.
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Now, the forward relationship is the easiest, and it's as simple as creating a constant called items and
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it's going to be set to a collection type called List.
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And this is something that comes from Realm.
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And it's basically a container type just as array or dictionaries are container types.
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But this is something that they decide called List which is very, very similar to a raise.
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Now, if we create a new list, we also have to specify what is the data type of the items inside the List
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or inside the array.
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And in our case, it's going to be a whole bunch of Item objects.
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So this syntax may or may not be familiar to you, but when we create new arrays, so let's say, let's create
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let array equals, I don't know, 1, 2, 3.
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This is one way of declaring the array,
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so this uses type inference,
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right?
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But the other way of doing it.
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We could also say that this is an array of integers and we're going to initialize it as an empty array.
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And the other way still is that we can simply say, the array is going to be of type an array of integers,
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and it's going to be equal to one two three.
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That's perfectly valid.
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Now, a method that we haven't seen so far is we can also declare this as the type Array and say that
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it is going to contain an array of integers.
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Now, this and the previous syntax are both valid.
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So if you wanted to declare a empty array of integers, you could also do it like so by simply initializing
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it as an empty array of integers.
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Now, this is exactly what we're going to do here.
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We're going to create this constant called Items and it's going to hold a list of item objects and we're
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going to initialize it as an empty list.
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So I'm just trying to show that this is not some foreign sort of syntax.
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It's actually something that's perfectly Swifty.
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And the only thing that's foreign is this class List which is something that comes from our Realm framework.
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So this, essentially, defines the forward relationship, i.e., inside each Category,
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there's this thing called items that's going to point to a list of Item objects.
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Now in Realm, the inverse relationship is not defined automatically.
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Instead, you actually have to go inside the Item class and create it yourself.
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So let's do that now.
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So we called our inverse relationship parentCategory and it's going to be set to an object of the class
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LinkingObjects. And LinkingObjects are auto-updating containers that represent zero or more objects
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that are linked to its owning model object through a property relationship.
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Now that sounds really, really complicated, but it's actually not. It's simply defining the inverse relationship
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of items.
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So each category has a one to many relationship with a List of I tems and each item has an inverse relationship
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to a category called the parentCategory.
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And we have to initialize LinkingObjects with a type and a property name.
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So the type is going to be whatever object that we have .type.
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So if we simply wrote Category, then this is just the class.
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It's not actually the type.
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In order to make it the type of Category, we have to say .self.
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And that becomes the Category type.
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And then the property that we need to specify is what is the name of that forward relationship which
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is, of course, items.
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So all we have to say here is specify a string called "items" that points to the forward relationship.
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So, now we have the forward relationship,
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each Category having a list of items. And the reverse category, each item has a parentCategory that is
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of the type Category
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and it comes from that property
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called "items."
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So now we're ready to go and fix all of these errors and use our Realm data model, instead of our Core
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Data data model.
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So let's head over to CategoryViewController and we're going to, first, initialize a new Realm.
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So we're gonna say let realm = try! to initialize a new Realm.
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Now I know that we've always been cautious with the exclamation marks, as you should be, but not all cases
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of try exclamation mark are bad.
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So this is what you would call a code smell or bad smell, and it's something that indicates or hints
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at possibly bad code, and maybe a deeper problem, but not always.
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So just as while you're walking around and you smell something that smells really bad, it could be a dead
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body or it could be just the trash,
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right?
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And in this case, it's actually not an issue.
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The reason why this initialization throws or can throw an error is because according to Realm, the first
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time when you're creating a new Realm instance, it can fail if your resources are constrained.
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But in practice, this can only happen the first time a Realm instance is created on a given thread.
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So if you look at the Realm documentation, then this is a perfectly valid way of declaring and creating
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a new Realm.
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So if you're interested in that, then I'll link to the documentation and you can have a deeper read.
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Xcode is still giving us this error saying "Use of unresolved identify 'Realm,'" and that is because,
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of course, that we've got to use Realm, instead of Core Data.
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So I'm just going to replace that which is going to create more errors for me,
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but we're going to go and fix those steadily one by one.
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Let's go into the parts of our code where we create a new piece of data inside our database and that
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is, of course, inside the addButtonPressed and it's--and more specifically, it's in this closure when
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we press the Add button in that UIAlert.
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So whereas currently, we're creating this new category which was initialized using a context because
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we were using Category as a NSManagedObject.
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But now our category is just going to be a straight up object from the Category data model.
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And the new category also has a property code name because we defined it over here, and that's going
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to be set to equal the text that's currently inside the textField of the UIAlert.
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And finally, all we have to do is change this saveCategories method, because at the moment, it tries to
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save the context. Whereas what we need to do is to try to write to our Realm, so try realm.write
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and we're going to try to say realm.add category, and that is going to be the new category,
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right?
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So that will need to be passed in to this method.
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So let's replace this method to be called save and it's going to take something called a Category, and
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that is going to be of type Category, and that will then get passed in here,
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and we now have to fix the part where we call that method. And we're going to say save category is going
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to be the newCategory that we just created over here.
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So, now if we hit command B to refresh and try to get rid of some of the errors that don't actually belong
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here, then our IBAction is looking pretty clean.
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Now, we've still got some other errors in this code and also in our TodoListViewController. But I'm
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going to simply comment out all of this and I'm gonna go into the TodoListViewController and find
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those errors, and we're also going to comment that out, and I'm going to comment out this particular function,
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as well as this entire extension.
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And finally, we're going to also come on out the loaItems and I'm going to hit run to see if we can
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build our app.
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Okay.
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So currently, all that our code should be able to do is when we press the addButtonPressed, we should
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get a UIAlert, and if we type some text into the UIAlert, then it should create a new object from the
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class Category, and we should be able to save that category into our Realm container by using realm.write
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and realm.add.
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So let's give it a go.
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Let's press Add and let's create a new category called Home and add one called Work. And if we have a look
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inside our Realm Browser, then you can see that we've got three data models now. So Data is a little bit
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of a relic back from when we created it in our App Delegate. An item is empty because we've yet to implement
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any code inside the TodoListViewController.
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But our Category model has two new objects, one with the name Home, one with the name Work, and they have
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an associated list of items.
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Now, the list currently is zero, but as soon as we start assigning some items to that list, then these will
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be populated as we'll see a little bit later on.
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But what we can confirm is that our code worked and we were able to save and persist our categories.
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So now all that's left to do is to load our data, not from our context or Core Data, but from our realm.
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So for all of that and more, I'll see you on the next lesson.