Adding a One-to-Many Entity Relationship

[MUSIC] 0:00 Our entity data model currently only contains a single entity, ComicBook. 0:04 While the ComicBook entity is an important part of our model, 0:10 it's not enough on its own to fully describe our data. 0:13 As we've previously discussed, each ComicBook belongs to a series. 0:17 Our ComicBook entity class currently contains a SeriesTitle property that, as 0:22 the name suggests, contains the title of the series that the comic book belongs to. 0:27 If we want to also have a description for 0:32 the series, we can add another property to the ComicBook entity, SeriesDescription. 0:34 This would work, but 0:39 now the ComicBook entity has two properties related to a series. 0:41 And if we have more than one ComicBook object instance that belongs to the same 0:45 series, the series title and series description property string values 0:49 would be repeated within each ComicBook instance. 0:54 To avoid this unnecessary duplication of values, 0:58 we can add a Series entity to our model. 1:01 The Series entity will have ID, Title and Description properties. 1:04 Then we can replace the ComicBook entity series title 1:09 into series description properties with a reference to the series entity. 1:12 The relationship between a ComicBook and a series is known as a One-to-Many or 1:16 Many-to-One, depending on your point of view. 1:21 From the series point of view, a series contains one or more, or many comics. 1:24 And from the ComicBook's point of view, a ComicBook belongs to a single or 1:30 one series. 1:35 So from the series point of view, their relationship is One-to-Many. 1:37 And from the ComicBook's point of view, it's Many-to-One. 1:41 In addition to the Many-to-One relationship, 1:45 we can model Many-to-Many and One-to-One relationships. 1:47 We'll implement a Many-to-Many relationship later in the section. 1:51 For more information on how to model a One-to-One, or 1:55 One-to-Zero or One relationship, see the teacher's notes. 1:58 Let's add a series entity to our model and 2:03 define the relationship between the ComicBook and series entities. 2:06 Right click on the Models folder and select Add > Class. 2:10 Name the class Series and click Add. 2:16 Add the public access modifier and 2:23 the following properties, an int Property named Id. 2:26 This property will contain a unique value for each series in our system. 2:32 A string property named Title. 2:38 This is the title of the series. 2:41 A string property named Description. 2:45 This is a description of the series. 2:50 Now that we have the series entity class, 2:52 let's update the ComicBook entity class to make use of it. 2:55 I'll replace the series title property with the series property of type series. 3:04 By adding this property to the ComicBook entity class, 3:12 we've defined a Many-to-One relationship between the ComicBook and series entities. 3:15 Each ComicBook instance can be associated with exactly one series instance, 3:21 but there's no restriction with how many ComicBook instances 3:26 a series instance can be associated with. 3:29 In this relationship, the series entity is the principal and the ComicBook entity 3:32 is the dependent, meaning that a ComicBook is dependent upon a series. 3:38 EF refers to the series property as a navigation property, as it allows 3:43 us to navigate the relationship from the ComicBook entity to the series entity. 3:48 Including an Id property is important to do, so EF will know 3:55 that we intend the data for the series entity to be stored in its own table. 4:00 If we didn't include the id property, 4:05 EF would treat the series class as a complex type. 4:07 And its title and 4:11 description properties would be added to the ComicBook's database table. 4:12 Complex types give you a way with an EF to group related sets of properties 4:16 within an entity or across entities. 4:21 For more information about EF complex types, see the teacher's notes. 4:25 We can also define an navigation property on the series entity, 4:30 though doing so is optional. 4:34 Since a series can be associated with more than one ComicBook, 4:35 we need to define the navigation property as a collection, 4:41 public ICollection of type <ComicBook> ComicBooks get; set for the property type. 4:47 I prefer to use the ICollection generic interface instead of a concrete type like 4:55 list of T, in order to expose the functionality that I want to provide 5:00 without tying myself to a specific collection implementation. 5:04 We also need to initialize the collection in a default constructor. 5:09 public Series, then ComicBooks 5:14 = new List<ComicBook>. 5:19 This ensures that the ComicBooks property is ready to be used immediately after 5:26 instantiating a series entity object. 5:31 Now that we've updated our entities, 5:34 we need to update the program classes main method to get our app compiling again. 5:36 After the break, we'll make those changes and 5:41 review how our entity relationship has affected our database. 5:44