Transcript Software University

EF Code First (Advanced)
Automatic and Manual Migrations
SoftUni Team
Technical Trainers
Software University
http://softuni.bg
Table of Contents
1. Initialization Strategy
2. Automatic Migrations
3. Manual Migrations
4. Database Seeding
5. Migrations for Production
2
Questions
sli.do
#Entity
3
Code First Model Changes
Design During Initial Development
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
5
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>());
}
6
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
7
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>
{
Override method
Extend initializer
protected override void Seed(MyDBContext context)
{
// TODO Create and insert sample records
base.Seed(context);
}
}
Resume behavior
of base class
8
Database Migrations
Code First Migrations in Entity Framework
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
10
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:
Enable-Migrations
 This creates an Initial Migration
 For additional parameters (e.g. to change Context) see:
get-help Enable-Migrations -detailed
11
Automatic Migrations
 To enable Automatic Migrations, enter in the console:
Enable-Migrations –EnableAutomaticMigrations
 When the code is changed, a migration will be created
 Apply changes to database trough the console:
Print resulting SQL
Update-Database -Verbose
 Some changes cannot be done trough automatic migrations
12
Configuring Migrations
 Tell Entity Framework to use automatic migrations from the
context constructor:
Database.SetInitializer(
new MigrateDatabaseToLatestVersion<MyDbContext,
Configuration>());
 Allow changes that remove existing columns:
This is a generated class,
public Configuration()
found under "Migrations"
{
this.AutomaticMigrationsEnabled = true;
this.AutomaticMigrationDataLossAllowed = true;
}
13
Manual Migrations
 Code-based migrations provide full control
 To create migration, enter in the console:
Add-Migration [Name]
 To apply migration:
Name is optional
Update-Database
 To apply specific migration (including going to previous version):
Update-Database –TargetMigration: <Name>
14
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
CreateIndex("Car",
"RegistrationNumber",
true,
Unique?
"IX_Car_BrandId");
Column name
(can be array)
Index name
15
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",
Data type
"Rating",
c => c.Int(
Column name
nullable: false,
defaultValue: 3));
Default value
16
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:
Enable multi-line
strings in C#
Sql(@"UPDATE dbo.Posts
SET Abstract = LEFT(Content, 100)
WHERE Abstract IS NULL");
 See the DbMigration documentation for all available commands
17
Seeding the Database
 During a migration we can seed the database with some data
using the Seed() method
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();
}
 This method will be run after the migration is complete
18
Production Scripts
 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!
Update-Database -script
 To create script for specific migration (even if non-pending):
Update-Database –sourcemigration <Name> -script
19
Summary
1. Initializers let you customize the workflow
2. Migrations provide an easy way to make
changes to the code and database
3. You can seed the database with testing data
after each change
20
EF Code First (Advanced)
?
https://softuni.bg/courses/
License
 This course (slides, examples, demos, videos, homework, etc.)
is licensed under the "Creative Commons AttributionNonCommercial-ShareAlike 4.0 International" license
 Attribution: this work may contain portions from

"Databases" course by Telerik Academy under CC-BY-NC-SA license
Free Trainings @ Software University
 Software University Foundation – softuni.org
 Software University – High-Quality Education,
Profession and Job for Software Developers

softuni.bg
 Software University @ Facebook

facebook.com/SoftwareUniversity
 Software University Forums
 forum.softuni.bg