561 words (approximately 3 mins)

Notes from Razor Pages with Entity Framework Core in ASP.NET Core - Tutorial 1 of 8 | Microsoft Learn

Creating a Database

Program.cs

using (var scope = app.Services.CreateScope())
{
    var services = scope.ServiceProvider;

    var context = services.GetRequiredService<SchoolContext>();
    context.Database.EnsureCreated();
    // DbInitializer.Initialize(context);
}

The EnsureCreated method checks if the database exists. If not, it creates the database and schema.

This allows for the following workflow for data model changes:

  1. Delete the database
  2. Change the data model (ie: add an EmailAddress field)
  3. Run the app
  4. EnsureCreated will create a database with the new schema

When data exists that cannot be lost, use migrations instead of EnsureCreated.

Connection Strings

Connection strings are passed to DbContexts by calling a method on a DbContextOptions object.
In local development, ASP.NET Core reads the string from appsettings.json or appsettings.Development.json.

Primary Keys

By default, EF Core interprets a property that’s named ID or *classname*ID as the primary key.

Navigation Properties

Navigation properties are properties in a model that hold other entities related to this entity. For example, a Student may have a navigation property of Enrollments to hold all the Enrollment entities for that Student.

EF Core interprets a property as a foreign key if it’s named <navigation property name><primary key property name>. For example, StudentID is the foreign key for the Student navigation property, since the Student entity’s primary key is ID.

  • Foreign key properties can also be named <primary-key-property-name> such as CourseID since the Course entity’s primary key is CourseID.

Seeding a Database

Convention: create Data/DbInitializer.cs with code to seed a database with test data.

Scaffolding

EF Core’s scaffolding engine does not support nullable reference types—the models used in scaffolding cannot use them either.
Scaffolding will automatically register the Context class it creates with DI.

Database Context (DbContext)

The database context class derives from Microsoft.EntityFrameworkCore.DbContext and specifies which entities are included in the data model. It is the main class that coordinates EF Core functionality for a given data model.

This class lives in Data/*Some*Context.cs. This service is registered with DI.

Entity Set (DbSet)

An entity set typically corresponds to a database table.
An entity corresponds to a row in the table.

Entity States

The DbContext keeps track of the sync state between entities in memory and their corresponding rows in the database. An entity has these states:

  • Added—entity does not yet exist in database; SaveChanges will issue an INSERT statement
  • Unchanged—no changes to this entity
  • Modified—some or all of the entity’s properties have been modified; SaveChanges issues an UPDATE statement
  • Deleted—entity marked for deletion; SaveChanges issues a DELETE statement
  • Detached—entity is not being tracked by database context

Visual Studio

Visual Studio can view databases: View > SQL Server Object Explorer (SSOX)

Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore

This package provides ASP.NET Core middleware for EF Core error pages.

Asynchronous Programming in EF Core

EF Core uses async operations by default. However:

  • Only statements that cause queries or commands to be sent to the database are executed asynchronously.
  • An EF Core context is not thread safe — do not execute parallel operations.

Reading Entities

To read a single entity, prefer FirstOrDefaultAsync (which returns null if nothing is found) over:

  • SingleOrDefaultAsync which throws an exception if there’s more than one entity that satisfies the query filter
  • FindAsync because you cannot use Include with this method.

However, when you do not have to use Include for related data, prefer FindAsync as it is more efficient.