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Populating Our Database with Seed Data
7:31Let's see how we can use Code First Migrations to seed our database with lookup and test data.
We have our database, but
now we need to populate it was some data.
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This is commonly referred
to as seeding a database.
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Seed data can be faked or dummy data
to assist us with the development and
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testing of our application.
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Or it can be data that the application
requires in order to operate properly.
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Our comic book gallery model requires us
to specify the role that an artist has for
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a comic book that they
are associated with.
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To do this, we need the role table
to be populated with a set of values
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that we can select from.
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This kind of data is sometimes
referred to as Lookup Data.
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In the comic book artist table,
we have a role ID column
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which provides us with a primary key
value for a row in the role table.
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With that primary key value in hand,
we can query or
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look up the associated
role name column value.
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Let's see how we can use code first
migrations to seed our database.
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Our configuration class inherits
from the code first migrations,
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DbMigrationsConfiguration base class,
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which provides a seed method that we can
override and use to seed our database.
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The configuration class
that was generated for
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us when we enabled migrations
has already done this.
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The seed method has passed
an instance of our context class
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which we can use to reference
the roles Db set property.
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The Db set class contains
an outer update method
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that can be used to only insert data
if it doesn't exist in the database.
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If the data is already in the database
then an update will be performed.
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Developers commonly referred to
this operation as an up cert.
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This is helpful as the seed method will be
called every time after a migration has
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been applied to the database using
the Update-Database command.
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The first parameter that we need
to pass to the outer update method
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is a lambda expression that specifies
the properties, that EF should
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use when determining whether an add or
update operation should be performed.
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Each row should have a unique name.
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So, we can use the name property as
a natural key, r goes to r.Name.
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Then, we can pass one or more row
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entity object instances to add or
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update, new Role, Name = "Script".
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Don't forget to add a using directive for
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the ComicBookGalleryModel.Models
namespace.
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Then, let's add a second role,
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new Role, Name = "Pencils".
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The context's save changes method
is automatically called, so
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there's no need to call it
directly from the seed method.
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Now, let's run our application.
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Make sure you have a breakpoint
set in the program.cs file
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on the query that retrieves
the list of comic books.
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We're at our breakpoint.
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Press F5 and
then Enter to continue execution.
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Now let's use the SQL Server Object
Explorer Window to view the data in
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the role table.
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Right-click on the role table and
select view data.
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And the table doesn't contain any data,
why is this?
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Changes that you make to
the seed method won't take effect
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until a migration is applied using
the update-database command.
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We don't have a migration to apply,
but that's okay.
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We can still run the update-database
command, which will determine that there
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aren't any pending migrations to apply,
and then call the seed method.
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Update-database.
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We can see here in the console output
that the seed method was called.
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Right-click on the role table and
select view data again.
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And now we're seeing our C data.
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Let's look at an example
of adding some test data
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that utilizes our role lookup data.
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To save us some time, I already have
our C data code here in notepad.
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If you're following along, you can find a
link to this code in the teacher's notes.
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I'll select all,
copy to the clipboard, and
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then paste into the seed method
replacing our previous implementation.
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Notice that we've updated our Role C data,
to use the roleId property for
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the property that EF should use to
determine if this should be an add or
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update.
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And we updated the role entity
object instances to have explicit
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Id property values, which are set to
the constant integer values defined here.
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By switching to this approach we now
have easy access to the roleId values
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when associating an artist
with a comic book.
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Which we'll see an example
of in just a bit.
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Here we're adding two artists,
Stan Lee and Steve Ditko, and
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a little further down we're adding
a series, The Amazing Spider-Man.
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And finally, here's where we're
adding our first comic book.
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Notice how we're making use of the
available foreign key SeriesId property to
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associate the series.
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And, here we're calling the comic
book entities AddArtist method,
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which allows us to pass the artistId to
add along with the roleId for their role.
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We put our cursor on the AddArtist method
and press alt F12 to pick the definition.
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We can see that the parameter values
flow to the corresponding comic book
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artist entity class
foreign key properties.
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Using the package manager console, let's
run the update-database command to update
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our database with our new test data.
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Update-database and press enter,
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and run our app again.
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Press F5 to continue execution.
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And here's our test data
displayed in the console.
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Press enter to continue execution.
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