1. EF Code First (Advanced)
Automatic and Manual Migrations
Code
First
SoftUni Team
Technical Trainers
Software University
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2. Table of Contents Initialization Strategy Automatic Migrations
Manual Migrations
Database Seeding
Migrations for Production
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3. sli.do #Entity Questions
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4. Code First Model Changes
Design During Initial Development

5. Changes in Domain Classes
What happens when we change our models?
Entity Framework compares our model with the model from the __MigrationHistory table in the DB
By default Entity Framework only creates the database
EF doesn't do any schema changes after that

6. Database Initialization Strategy
You can drop and recreate the database if changes are made
May be the most efficient method during early development
Change the DB Initialization strategy from the constructor of your context:
public MyDBContext() : base("MyConnectionString") {
Database.SetInitializer(
new DropCreateDatabaseIfModelChanges<MyDBContext>()); }

7. Initialization Strategies
CreateDatabaseIfNotExists
Default migration
DropCreateDatabaseIfModelChanges
We lose all the data when the model changes
DropCreateDatabaseAlways
Great for automated integration testing
MigrateDatabaseToLatestVersion
This option uses our migrations
Custom strategy
Implement IDatabaseInitializer for custom migration strategy

8. Database Seeding
You can place sample data in the DB every time it's recreated
Override the Seed method in an extended strategy class:
public class MyDBInitializer
: DropCreateDatabaseAlways<MyDBContext> {
protected override void Seed(MyDBContext context)
// TODO Create and insert sample records
base.Seed(context); }
Override method
Extend initializer
Resume behavior
of base class

9. Code First Migrations in Entity Framework
Database Migrations
Code First Migrations in Entity Framework

10. Incremental Changes
We can't always recreate the database when changes arise
Using Code First Migrations we can handle differences between models and database in a non-destructive way
Preserve existing data
Share changes trough Source Control
Allow rapid deployment in a production environment

11. Code First Migrations in Visual Studio
To enable Migrations, open the Package Manager Console
Tools  NuGet Package Manager  Package Manager Console
Enter the following command:
This creates an Initial Migration
For additional parameters (e.g. to change Context) see:
Enable-Migrations
get-help Enable-Migrations -detailed

12. Automatic Migrations
To enable Automatic Migrations, enter in the console:
When the code is changed, a migration will be created
Apply changes to database trough the console:
Some changes cannot be done trough automatic migrations
Enable-Migrations –EnableAutomaticMigrations
Print resulting SQL
Update-Database -Verbose

13. Configuring Migrations
Tell Entity Framework to use automatic migrations from the context constructor:
Allow changes that remove existing columns:
Database.SetInitializer(
new MigrateDatabaseToLatestVersion<MyDbContext,
Configuration>());
This is a generated class, found under "Migrations"
public Configuration() {
this.AutomaticMigrationsEnabled = true;
this.AutomaticMigrationDataLossAllowed = true; }

14. Manual Migrations Code-based migrations provide full control
To create migration, enter in the console:
To apply migration:
To apply specific migration (including going to previous version):
Add-Migration [Name]
Name is optional
Update-Database
Update-Database –TargetMigration: <Name>

15. Custom Migration Mappings
Two methods are created for each migration:
Up() – update database to current model
Down() – go back to previous version
You can add custom mappings to the Up method
To add an index:
Table name
Column name
(can be array)
CreateIndex("Car",
"RegistrationNumber",
true,
"IX_Car_BrandId");
Unique?
Index name

16. Custom Migration Mappings (2)
When creating non-nullable column, EF will assign the language specific initial value
To assign custom default value:
Table name
AddColumn("dbo.Blogs",
"Rating",
c => c.Int(
nullable: false,
defaultValue: 3));
Data type
Column name
Default value

17. Custom Migration Mappings (3)
Some times it's best to use raw SQL to update the database
To execute an SQL statement in a migration:
See the DbMigration documentation for all available commands
Enable multi-line strings in C#
dbo.Posts
SET Abstract = LEFT(Content, 100)
WHERE Abstract IS NULL");

18. Seeding the Database
During a migration we can seed the database with some data using the Seed() method
This method will be run after the migration is complete
protected override void Seed(ForumContext context) {
/* This method will be called after migrating to the latest version.
You can use the DbSet<T>.AddOrUpdate() helper extension method
to avoid creating duplicate seed data. E.g. */
context.Tags.AddOrUpdate(t => t.Text, new Tag { Text = "C#" });
context.SaveChanges(); }

19. Production Script s
You can generate an SQL scripts to be run on the production server by the DBA
Note this has to be run before the migration is applied locally!
To create script for specific migration (even if non-pending):
Update-Database -script
Update-Database –sourcemigration <Name> -script

20. Summary Initializers let you customize the workflow
Migrations provide an easy way to make changes to the code and database
You can seed the database with testing data after each change
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21. EF Code First (Advanced)
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22. License
This course (slides, examples, demos, videos, homework, etc.) is licensed under the "Creative Commons Attribution- NonCommercial-ShareAlike 4.0 International" license
Attribution: this work may contain portions from
"Databases" course by Telerik Academy under CC-BY-NC-SA license
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