By [Tom Dykstra](https://github.com/tdykstra), [Jon P Smith](https://twitter.com/thereformedprog), and [Rick Anderson](https://twitter.com/RickAndMSFT)
When a new app is developed, the data model changes frequently. Each time the model changes, the model gets out of sync with the database. This tutorial started by configuring the Entity Framework to create the database if it doesn't exist. Each time the data model changes:
This approach to keeping the DB in sync with the data model works well until you deploy the app to production. When the app is running in production, it's usually storing data that needs to be maintained. The app can't start with a test DB each time a change is made (such as adding a new column). The EF Core Migrations feature solves this problem by enabling EF Core to update the DB schema instead of creating a new DB.
## Entity Framework Core NuGet packages for migrations
To work with migrations, 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 Core 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. **Note:** This package must be installed by editing the *.csproj* file. The`install-package` command or the package manager GUI cannot be used to install this package. Edit the *.csproj* file by right-clicking the project name in **Solution Explorer** and selecting **Edit ContosoUniversity.csproj**.
The version numbers in the preceding example were current when the tutorial was written. Use the same version for the EF Core CLI tools found in the other packages.
Changing the DB name in the connection string causes the first migration to create a new DB. A new DB is created because one with that name doesn't exist. Changing the connection string isn't required for getting started with migrations.
User profile is available. Using 'C:\Users\username\AppData\Local\ASP.NET\DataProtection-Keys' as key repository and Windows DPAPI to encrypt keys at rest.
Entity Framework Core 2.0.0-rtm-26452 initialized 'SchoolContext' using provider 'Microsoft.EntityFrameworkCore.SqlServer' with options: None
Done. To undo this action, use 'ef migrations remove'
```
If the migration fails with the message "*cannot access the file ... ContosoUniversity.dll because it is being used by another process.*" is displayed:
* Stop IIS Express.
* Exit and restart Visual Studio, or
* Find the IIS Express icon in the Windows System Tray.
* Right-click the IIS Express icon, and then click **ContosoUniversity > Stop Site**.
The EF Core command `migrations add` generated code to create the DB from. This migrations code is in the *Migrations\<timestamp>_InitialCreate.cs* file. The `Up` method of the `InitialCreate` class creates the DB tables that correspond to the data model entity sets. 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.
The preceding code is for the initial migration. That code was created when the `migrations add InitialCreate` command was run. The migration name parameter ("InitialCreate" in the example) is used for the file name. The migration name can be any valid file name. It's best to choose a word or phrase that summarizes what is being done in the migration. For example, a migration that added a department table might be called "AddDepartmentTable."
If the initial migration is created and the DB exits:
* The DB creation code doesn't need to run because the DB already matches the data model. If the DB creation code is run, it doesn't make any changes because the DB already matches the data model.
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](https://docs.microsoft.com/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 output. In the preceding command, logs for the SQL commands that set up the DB are displayed. Most of the logs are omitted in the following sample output:
User profile is available. Using 'C:\Users\username\AppData\Local\ASP.NET\DataProtection-Keys' as key repository and Windows DPAPI to encrypt keys at rest.
To reduce the level of detail in log messages, change the log levels in the *appsettings.Development.json* file. For more information, see [Introduction to logging](xref:fundamentals/logging/index).
Use **SQL Server Object Explorer** to inspect the DB. Notice the addition of an `__EFMigrationsHistory` table. The `__EFMigrationsHistory` table keeps track of which migrations have been applied to the DB. View the data in the `__EFMigrationsHistory` table, it shows one row for the first migration. The last log in the preceding CLI output example shows the INSERT statement that creates this row.
We recommend production apps should **not** call [Database.Migrate](https://docs.microsoft.com/dotnet/api/microsoft.entityframeworkcore.relationaldatabasefacadeextensions.migrate?view=efcore-2.0#Microsoft_EntityFrameworkCore_RelationalDatabaseFacadeExtensions_Migrate_Microsoft_EntityFrameworkCore_Infrastructure_DatabaseFacade_) at application startup. `Migrate` shouldn't be called from an app in server farm. For example, if the app has been cloud deployed with scale-out (multiple instances of the app are running).
The EF Core commands for the PMC are in the [Microsoft.EntityFrameworkCore.Tools](https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.Tools) package. This package is included in the [Microsoft.AspNetCore.All](xref:fundamentals/metapackage) metapackage, so you don't have to install it.
**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`.
For more information about the CLI commands, see [.NET Core CLI](https://docs.microsoft.com/ef/core/miscellaneous/cli/dotnet).
For more information about the PMC commands, see [Package Manager Console (Visual Studio)](https://docs.microsoft.com/ef/core/miscellaneous/cli/powershell).
