The Contoso University sample web application demonstrates how to create ASP.NET Core 1.1 MVC web applications using Entity Framework Core 1.1 and Visual Studio 2017. For information about the tutorial series, see [the first tutorial in the series](intro.md).
In this tutorial, you start using the EF Core migrations feature for managing data model changes. In later tutorials, you'll add more migrations as you change the data model.
## Introduction to migrations
When you develop a new application, your data model changes frequently, and each time the model changes, it gets out of sync with the database. You started these tutorials by configuring the Entity Framework to create the database if it doesn't exist. Then each time you change the data model -- add, remove, or change entity classes or change your DbContext class -- you can delete the database and EF creates a new one that matches the model, and seeds it with test data.
This method of keeping the database in sync with the data model works well until you deploy the application to production. When the application is running in production it is usually storing data that you want to keep, and you don't want to lose everything each time you make a change such as adding a new column. The EF Core Migrations feature solves this problem by enabling EF to update the database schema instead of creating a new database.
## Entity Framework Core NuGet packages for migrations
To work with migrations, you can use the **Package Manager Console** (PMC) or the command-line interface (CLI). These tutorials show how to use CLI commands. Information about the PMC is at [the end of this tutorial](#pmc).
The EF tools for the command-line interface (CLI) are provided in [Microsoft.EntityFrameworkCore.Tools.DotNet](https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.Tools.DotNet). To install this package, add it to the `DotNetCliToolReference` collection in the *.csproj* file, as shown. (The version numbers in this example were current when the tutorial was written.)
In the *appsettings.json* file, change the name of the database in the connection string to ContosoUniversity2 or some other name that you haven't used on the computer you're using.
This change sets up the project so that the first migration will create a new database. This isn't required for getting started with migrations, but you'll see later why it's a good idea.
> [!NOTE]
> As an alternative to changing the database name, you can delete the database. Use **SQL Server Object Explorer** (SSOX) or the `database drop` CLI command:
If you see an error message *No executable found matching command "dotnet-ef"*, see [this blog post](http://thedatafarm.com/data-access/no-executable-found-matching-command-dotnet-ef/) for help troubleshooting.
If you see an error message "*cannot access the file ... ContosoUniversity.dll because it is being used by another process.*", find the IIS Express icon in the Windows System Tray, and right-click it, then click **ContosoUniversity > Stop Site**.
When you executed the `migrations add` command, EF generated the code that will create the database from scratch. This code is in the *Migrations* folder, in the file named *<timestamp>_InitialCreate.cs*. The `Up` method of the `InitialCreate` class creates the database tables that correspond to the data model entity sets, and the `Down` method deletes them, as shown in the following example.
Migrations calls the `Up` method to implement the data model changes for a migration. When you enter a command to roll back the update, Migrations calls the `Down` method.
This code is for the initial migration that was created when you entered the `migrations add InitialCreate` command. The migration name parameter ("InitialCreate" in the example) is used for the file name and can be whatever you want. It's best to choose a word or phrase that summarizes what is being done in the migration. For example, you might name a later migration "AddDepartmentTable".
If you created the initial migration when the database already exists, the database creation code is generated but it doesn't have to run because the database already matches the data model. When you deploy the app to another environment where the database doesn't exist yet, this code will run to create your database, so it's a good idea to test it first. That's why you changed the name of the database in the connection string earlier -- so that migrations can create a new one from scratch.
Because the current database schema is represented in code, EF Core doesn't have to interact with the database to create migrations. When you add a migration, EF determines what changed by comparing the data model to the snapshot file. EF interacts with the database only when it has to update the database.
The snapshot file has to be kept in sync with the migrations that create it, so you can't remove a migration just by deleting the file named *<timestamp>_<migrationname>.cs*. If you delete that file, the remaining migrations will be out of sync with the database snapshot file. To delete the last migration that you added, use the [dotnet ef migrations remove](https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/dotnet#dotnet-ef-migrations-remove) command.
Use **SQL Server Object Explorer** to inspect the database as you did in the first tutorial. You'll notice the addition of an __EFMigrationsHistory table that keeps track of which migrations have been applied to the database. View the data in that table and you'll see one entry for the first migration.
Run the application to verify that everything still works the same as before.
The EF tooling for managing migrations is available from .NET Core CLI commands or from PowerShell cmdlets in the Visual Studio **Package Manager Console** (PMC) window. This tutorial shows how to use the CLI, but you can use the PMC if you prefer.
[Microsoft.EntityFrameworkCore.Tools](https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.Tools) package. Unlike the CLI tools, you don't have to edit the *.csproj* file; you can install it by using the **Package Manager Console** or the **NuGet Package Manager** GUI.
For more information about the CLI commands, see [.NET Core CLI](https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/dotnet).
For more information about the PMC commands, see [Package Manager Console (Visual Studio)](https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/powershell).
In this tutorial, you've seen how to create and apply your first migration. In the next tutorial, you'll begin looking at more advanced topics by expanding the data model. Along the way you'll create and apply additional migrations.
