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.
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's 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.
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).
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 to get started with migrations, but you'll see later why it's a good idea.
> 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.
Migrations creates a *snapshot* of the current database schema in *Migrations/SchoolContextModelSnapshot.cs*. When you add a migration, EF determines what changed by comparing the data model to the snapshot file.
When deleting a migration, use the [dotnet ef migrations remove](/ef/core/miscellaneous/cli/dotnet#dotnet-ef-migrations-remove) command. `dotnet ef migrations remove` deletes the migration and ensures the snapshot is correctly reset.
The output from the command is similar to the `migrations add` command, except that you see logs for the SQL commands that set up the database. Most of the logs are omitted in the following sample output. If you prefer not to see this level of detail in log messages, you can change the log level in the *appsettings.Development.json* file. For more information, see <xref:fundamentals/logging/index>.
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 row for the first migration. (The last log in the preceding CLI output example shows the INSERT statement that creates this row.)
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.
The EF commands for the PMC commands are in the [Microsoft.EntityFrameworkCore.Tools](https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.Tools) package. This package is included in the [Microsoft.AspNetCore.App metapackage](xref:fundamentals/metapackage-app), so you don't need to add a package reference if your app has a package reference for `Microsoft.AspNetCore.App`.
**Important:** This isn't the same package as the one you install for the CLI by editing the *.csproj* file. The name of this one ends in `Tools`, unlike the CLI package name which ends in `Tools.DotNet`.
Advance to the next tutorial to begin looking at more advanced topics about expanding the data model. Along the way you'll create and apply additional migrations.
