Populating Our Database with Seed Data

We have our database, but now we need to populate it was some data. 0:00 This is commonly referred to as seeding a database. 0:04 Seed data can be faked or dummy data to assist us with the development and 0:08 testing of our application. 0:13 Or it can be data that the application requires in order to operate properly. 0:15 Our comic book gallery model requires us to specify the role that an artist has for 0:21 a comic book that they are associated with. 0:26 To do this, we need the role table to be populated with a set of values 0:28 that we can select from. 0:33 This kind of data is sometimes referred to as Lookup Data. 0:35 In the comic book artist table, we have a role ID column 0:39 which provides us with a primary key value for a row in the role table. 0:42 With that primary key value in hand, we can query or 0:47 look up the associated role name column value. 0:52 Let's see how we can use code first migrations to seed our database. 0:56 Our configuration class inherits from the code first migrations, 1:00 DbMigrationsConfiguration base class, 1:04 which provides a seed method that we can override and use to seed our database. 1:07 The configuration class that was generated for 1:12 us when we enabled migrations has already done this. 1:14 The seed method has passed an instance of our context class 1:20 which we can use to reference the roles Db set property. 1:24 The Db set class contains an outer update method 1:29 that can be used to only insert data if it doesn't exist in the database. 1:33 If the data is already in the database then an update will be performed. 1:38 Developers commonly referred to this operation as an up cert. 1:42 This is helpful as the seed method will be called every time after a migration has 1:46 been applied to the database using the Update-Database command. 1:51 The first parameter that we need to pass to the outer update method 1:58 is a lambda expression that specifies the properties, that EF should 2:02 use when determining whether an add or update operation should be performed. 2:06 Each row should have a unique name. 2:11 So, we can use the name property as a natural key, r goes to r.Name. 2:13 Then, we can pass one or more row 2:27 entity object instances to add or 2:32 update, new Role, Name = "Script". 2:37 Don't forget to add a using directive for 2:47 the ComicBookGalleryModel.Models namespace. 2:49 Then, let's add a second role, 2:55 new Role, Name = "Pencils". 3:01 The context's save changes method is automatically called, so 3:06 there's no need to call it directly from the seed method. 3:10 Now, let's run our application. 3:14 Make sure you have a breakpoint set in the program.cs file 3:16 on the query that retrieves the list of comic books. 3:20 We're at our breakpoint. 3:27 Press F5 and then Enter to continue execution. 3:29 Now let's use the SQL Server Object Explorer Window to view the data in 3:37 the role table. 3:42 Right-click on the role table and select view data. 3:43 And the table doesn't contain any data, why is this? 3:50 Changes that you make to the seed method won't take effect 3:56 until a migration is applied using the update-database command. 3:59 We don't have a migration to apply, but that's okay. 4:03 We can still run the update-database command, which will determine that there 4:07 aren't any pending migrations to apply, and then call the seed method. 4:10 Update-database. 4:14 We can see here in the console output that the seed method was called. 4:26 Right-click on the role table and select view data again. 4:32 And now we're seeing our C data. 4:39 Let's look at an example of adding some test data 4:42 that utilizes our role lookup data. 4:45 To save us some time, I already have our C data code here in notepad. 4:49 If you're following along, you can find a link to this code in the teacher's notes. 4:54 I'll select all, copy to the clipboard, and 4:58 then paste into the seed method replacing our previous implementation. 5:02 Notice that we've updated our Role C data, to use the roleId property for 5:25 the property that EF should use to determine if this should be an add or 5:30 update. 5:34 And we updated the role entity object instances to have explicit 5:36 Id property values, which are set to the constant integer values defined here. 5:41 By switching to this approach we now have easy access to the roleId values 5:47 when associating an artist with a comic book. 5:52 Which we'll see an example of in just a bit. 5:55 Here we're adding two artists, Stan Lee and Steve Ditko, and 6:00 a little further down we're adding a series, The Amazing Spider-Man. 6:05 And finally, here's where we're adding our first comic book. 6:11 Notice how we're making use of the available foreign key SeriesId property to 6:17 associate the series. 6:22 And, here we're calling the comic book entities AddArtist method, 6:27 which allows us to pass the artistId to add along with the roleId for their role. 6:32 We put our cursor on the AddArtist method and press alt F12 to pick the definition. 6:41 We can see that the parameter values flow to the corresponding comic book 6:46 artist entity class foreign key properties. 6:51 Using the package manager console, let's run the update-database command to update 6:55 our database with our new test data. 7:00 Update-database and press enter, 7:07 and run our app again. 7:12 Press F5 to continue execution. 7:18 And here's our test data displayed in the console. 7:23 Press enter to continue execution. 7:27