Entity Framework Core Series Tutorial-26-Data Migration

Entity Framework Core data migration

Migration is a way to keep the database architecture in sync with the EF Core model by retaining data.

As shown in the figure above, the EF Core API builds an EF Core model from domain (entity) classes, and EF Core migration will create or update a database schema based on the EF Core model. Every time you change the domain class, you need to run a migration to keep the database schema up to date.
EF Core migration is a set of commands that you can execute in the NuGet package manager console or dotnet command line interface (CLI).
The following table lists the important migration commands in EF Core.

Package Manager console commands	dotnet command line command	Instructions
add-migration <migration name>	Add <migration name>	Create a migration by adding a migration snapshot
Remove-migration	Remove	Delete the last migration snapshot
Update-database	Update	Update the database schema based on the last migration snapshot
Script-migration	Script	Generate SQL script using all migration snapshots

Add migration

For the first time, you defined the initial domain class. At this point, your application does not have a database to store data in your domain class. Therefore, first, you need to create a migration.

Open the package manager console from Tools-> NuGet Package Manager-> Package Manager Console in Visual Studio, and then execute the following command to add the migration.

add-migration MyFirstMigration

If you are using the dotnet command line interface, please execute the following command.

dotnet ef migrations add MyFirstMigration

In the above command, MyFirstMigration is the name of the migration. This will create three files in the project's "Migration" folder as shown below.

1. <timestamp> _ <migration name> .cs: main migration file, which contains the migration operations in the Up () and Down () methods The Up () method includes code for creating database objects, and the Down () method includes code for deleting database objects.
2. <Timestamp> _ <migration name> .Designer.cs: migration metadata file, which contains information used by EF Core.
3. <contextclassname> ModelSnapshot.cs: snapshot of the current model. This is used to determine the changes made when creating the next migration.
   Now, after creating the migration snapshot, it is time to create the database.

Create or update a database

Use the following commands to create or update the database schema.

1. Package Manager Console

Update-Database

2. dotnet command line

dotnet ef database update

The Update command will create a database based on the context and domain class and migration snapshots, these snapshots are created using the add-migration or add command.
If this is the first migration, a table named __EFMigrationsHistory will also be created, which will store the names of all migrations and when they will be applied to the database.

Delete migration

If the last migration was not applied to the database, you can delete it. Use the following remove command to delete the last migration file created and restore the model snapshot.

1. Package Manager Console

remove-migration

2. dotnet command line

dotnet ef migrations remove

The above command will delete the last migration and restore the model snapshot to the previous migration. Please note that if the migration has been applied to the database, it will throw the following exception.
Migration <migration name> has been applied to the database. Restore it and try again. If the migration has been applied to other databases, consider using the new migration to revert its changes.

Restore migration

Suppose you change the domain class and use the add-migration command to create a second migration named MySecondMigration, and use the Update command to apply the migration to the database. However, for some reason, you want to restore the database to its previous state. In this case, use the update-database <migration name> command to restore the database to the specified previous migration snapshot.

1. Package Manager Console

Update-database MyFirstMigration

2. dotnet command line

dotnet ef database update MyFirstMigration

The above command will restore the database based on the migration named MyFirstMigration and delete all the changes applied to the second migration named MySecondMigration. This will also delete the MySecondMigration entry from the __EFMigrationsHistory table in the database.

Note: This will not delete the migration files related to MySecondMigration. Use the remove command to remove it from the project.

Generate SQL script

Use the following command to generate a SQL script for the database.

1. Package Manager Console

script-migration

2. dotnet command line

dotnet ef migrations script

The above script command will include all migrated scripts by default. You can use the -to and -from options to specify the migration scope.

From the package manager console command

You can use the package manager console in Visual Studio to execute the migration commands in Entity Framework Core. Open the package manager console from Tools-> NuGet Package Manager-> Package Manager Console in Visual Studio to execute the following commands.

Package Manager Console	usage
Get-Help entityframework	Get help entityframework to display information about Entity Framework commands
Add-Migration	<Migration name> Create migration by adding migration snapshot
Remove-Migration	Delete the last migration snapshot
Update-Database	Update the database schema based on the last migration snapshot
Script-Migration	Generate SQL script using all migration snapshots
Scaffold-DbContext	Generate DbContext and entity type classes for the specified database. This is called reverse engineering
Get-DbContext	Get information about DbContext type
Drop-Database	Delete the database

Get-Help

get-help entityframework

Add-Migration Add migration

get-help entityframework

Remove-Migration

get-help entityframework

Update-Database

get-help entityframework

Script-migration script migration

get-help entityframework

scaffold-dbcontext scaffold database context

get-help entityframework

Get-DbContext

get-help entityframework

Drop-Database

get-help entityframework

Command line interface commands for migration

Use .NET Core Command List Interface to execute Entity Framework core commands. To use the .NET CLI, add the <DotNetCliToolReference Include = "Microsoft.EntityFrameworkCore.Tools.DotNet" Version = "2.0.0" /> under the <ItemGroup> node by editing the .csproj file of the .NET Core project.
Open a command prompt, then navigate to the root folder of the project, and enter dotnet ef --help to list the EF Core commands, as shown below.
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As you can see above, there are three main EF commands available: database, dbcontext and migration. The following table lists all EF commands and subcommands.

command	Subcommand	usage
Database	drop	Delete the database
	update	Update the database to the specified migration
DbContext	info	Get information about DbContext type
	list	List available DbContext types
	scaffold	Provide DbContext and entity type for database
Migration	add	Add new migration
	list	List available migrations
	remove	Delete the last migration
	script:	Generate SQL script from migration

Let's look at the available options for each command.

Database Drop

dotnet ef database drop

Database Update

dotnet ef database update

DbContext Info

dotnet ef dbcontext info

DbContext List

dotnet ef dbcontext list

DbContext Scaffold

dotnet ef dbcontext scaffold

Add

dotnet ef migrations add

List

dotnet ef migrations list

Remove

dotnet ef migrations remove

Script

dotnet ef migrations script