Creating Entities

As we've seen in an earlier video, adding an entity to the database 0:00 is as simple as adding it to it's respective dbset property. 0:04 And, calling the save changes method. 0:07 We can use that knowledge to implement the add comic book method. 0:10 If you're following along, see the teacher's notes for 0:14 a link to download the project files. 0:17 The add comic book method is in the repository class which is located in 0:20 the data folder. 0:24 To start, we need to get an instance of the context class by calling the get 0:35 context method within a using statement. 0:39 Using (Context context = GetContext()). 0:41 Let's pick the definition for 0:46 the GetContext method by placing the cursor 0:50 on the method call and pressing alt F12. 0:55 The GetContext method instantiates an instance of the context class and 1:03 sets its database log property to an anonymous method 1:08 that writes messages to the output window, using the Debug.WriteLine method. 1:12 Press the Escape key to close the peak definition window. 1:19 Now let's finish implementing the add comic book method. 1:24 First, call the context comic books DB sets add method, 1:27 passing in the comic book parameter. 1:31 Then call the context save changes method. 1:39 Let's test our changes by setting a break point just inside of the add comic 1:44 book method, and run in the app. 1:49 Enter A to add a comic book, then Enter one to select the Bones series four, 1:58 for the issue number, skip the description for now, 2:04 1/1/2016 for the published on date. 2:09 Skip the average rating, three for the artist and two for the role. 2:13 Here we are at our break point in In the Locals window notice that while the series 2:23 ID Foreign Key property value is three, the series navigation property is null. 2:27 As we discussed in an earlier video this is one of the advantages 2:33 of defining Foreign Key properties in our entities. 2:37 We can associate a related entity by just setting their foreign key property. 2:40 In fact, when adding entities, this is exactly the approach that we want to use. 2:45 If the series navigation property wasn't null when adding the comic book entity to 2:50 the context, we'd inadvertently add a duplicate series record to the database. 2:54 Let's look at an example. 3:00 We're still at our breakpoint. 3:02 I'll press F5 to complete the process of adding the comic book. 3:04 And enter q to quit the app. 3:07 I'll set the series navigation property to an instance of the series' entity. 3:16 Comicbook series equals new series 3:21 with an ID of three and a title of bone. 3:26 Now run the app. 3:34 And enter a to add another comicbook. 3:39 Enter 1 to select the bones series, 5 for the issue number, 3:42 2/1/2016 for the published on date, 3:47 3 for the artist in 2 for the row. 3:52 We'll add our break point again. 3:57 Press F5 to continue execution. 3:59 Enter A again to start adding another comic book. 4:03 Notice that the Bone series is now listed twice. 4:07 Go ahead and switch back to Visual Studio and stop the app. 4:12 If we open the sequel server object Explorer window, 4:18 expand the ComicBookLibraryManager database, and 4:22 tables and view the data for the series table, 4:28 we can see that we definitely have two rows now. 4:34 Why is this happening? 4:38 One of the features of the context is that it tracks the state of and 4:40 the changes to each entity that it has currently loaded. 4:44 This is called change tracking. 4:48 When we add an entity to the context using the DbSet.Add Method, 4:50 the entity is added to the context and its state is set to added. 4:54 We can verify that by retrieving the entity's entry in the context 5:01 using the entry method and checking the value with 5:04 the state property just before calling the Save Changes method. 5:07 Var comicBookEntry = context.Entry(comicBook). 5:16 Let's also retrieve the entry for the series entity. 5:22 Var seriesEntry = 5:28 context.Entry(comicBook.Series). 5:32 Remove the previous break point Set a new break point on the line of code 5:41 that's calling the Save Changes method and run the app. 5:45 Enter A to add another comic book, 1 to select the Bone series, 5:55 6 for the issue number 3/1/2016 for 6:02 the published on date, 3 for the artist, and 2 for 6:08 the role, In the Locals window, 6:13 we can see that the comic book entry state property is set to added. 6:18 That's the expected behavior. 6:22 What might not be expected, 6:25 is that the series entries state property is also set to added. 6:26 When adding an entity to the context by calling the D.B. 6:34 said add method, E.F. what not only set to entity state to added but 6:37 also each of the entities related or child entities. 6:42 When the Save Changes method is called, each of the entities in the context that 6:46 has a state of added will be inserted into the database. 6:50 That's true even for the Bone series entity that has an ID property value 6:54 which in this case is three that matches an existing record in the database. 6:59 The Id property value is simply ignored. 7:04 Let's fix this problem. 7:10 After calling the Add method let's check if the series property isn't null and 7:16 the series entity Id property value is greater than zero. 7:21 If comic.Book.Series does not equal null and 7:24 comicBook.Series.Id is greater than zero. 7:29 An Id property value greater than zero 7:36 indicates that the entity already exists in the database. 7:39 Then let's retrieve the entry for 7:42 the series entity instead state property to unchanged. 7:44 Context Entry comicBook Series. 7:48 State = EntityState.Unchanged. 7:53 Changing the entry state from added to unchanged 8:00 will prevent the entity from being inserted into the database. 8:03 This problem also exists for the artist collection. 8:08 While we want any of the comic book artist entities to be added, 8:11 we wouldn't want the related artist or role entities to be inadvertently added. 8:15 I'll leave adding that code as an exercise for you to complete on your own. 8:20 If you get stuck, see the teacher's notes for the solution. 8:24 Let's do some cleanup. 8:29 Go ahead and remove the code that's setting the comic book series property. 8:30 To remove the duplicate series data, let's delete the database. 8:36 So we'll get a clean baseline copy of our data the next time that we start our app. 8:40 So, how much defensive coding is really necessary. 8:49 Well, it depends on the situation. 8:52 If you're the only developer working on the project, 8:55 then maybe this level of defensive coding is unnecessary. 8:58 Instead of writing this code, you could just ensure that you exclusively use 9:01 the foreign key properties and leave the navigation properties set to null. 9:05 But it's hard to predict how a project will evolve or 9:10 how your data access layer might be used in the future. 9:13 Given that, the best option might be to make your code as flexible and 9:17 reliable as possible. 9:21