Implementing Seeding, Custom Conventions and Interceptors in EF Core 1.0

This post was written by Alina Popa, a software engineer on the .NET team.

Updated 10/5/2016: This post has been updated to use improved Entity Framework Core techniques which don’t rely on internal APIs.

Implementing Seeding, Custom Conventions and Interceptors in EF Core 1.0

Introduction

Entity Framework Core (EF Core) is a lightweight and extensible version of the Entity Framework (EF) data access technology which is cross-platform and supports multiple database providers. You can find a comparison of EF Core vs. EF6 under the Entity Framework documentation.

When moving an application from EF6 to EF Core, you may encounter features that existed in EF6 but either are not present or are not yet implemented in EF Core. For many of those features, however, you can implement equivalent functionality.

Seeding

With EF6 you can seed a database with initial data by overriding one of the following Seed() methods:

EF Core does not provide similar APIs, and database initializers also no longer exist in EF Core. To seed the database, you would put the database initialization code in the application startup. If you are using migrations, call context.Database.Migrate(), otherwise use context.Database.EnsureCreated()/EnsureDeleted().

The patterns for seeding the database are discussed in issue 3070 in the Entity Framework Core repository on GitHub. The recommended approach is to run the seeding code within a service scope in Startup.Configure():

You can find here an example of database initialization that uses migrations, along with an implementation example of EnsureSeedData() method. The MusicStore sample also uses this pattern for seeding.

Please note that, in general, it is recommended to apply these operations manually (rather than performing migrations and seeding automatically on startup), to avoid racing conditions when there are multiple servers, and unintentional changes.

Custom Conventions

In Entity Framework 6 we can create custom configurations of properties and tables by using model-based conventions. For example, the following code in EF6 creates a convention to throw an exception when the column name is longer than 30 characters:

EF Core does not provide the IStoreModelConvention interface; however, we can create this convention by accessing the data model inside the OnModelCreating() method:

Note that the model is not read-only and it can be modified inside the loop.

Interceptors

Among other useful things, Entity Framework 6 provides the ability to intercept a context using IDbCommandInterceptor. Interceptors let you to get into the pipeline just before and just after a query or command is sent to the database.

Entity Framework Core doesn’t have any interceptors yet, but an important subset of the functionality of the interceptors can be achieved by using simple patterns, such as overriding DbContext.SaveChanges:

Some notes on the example above:

The call to ChangeTracker.DetectChanges() is to ensure that the change tracker is aware of the changes made to the entities, e.g. if you set .Category to a new Category on an existing Product, the new Category wouldn’t be tracked until DetectChanges() is called or it’s added explicitly through DbSet or ChangeTracker.
Setting AutoDetectChangesEnabled to false before calling the base SaveChanges is for performance reasons, to avoid calling DetectChanges() again.

Interceptors and seeding are high on the feature backlog and the Entity Framework team plans to address them in the near future.