Getting Started with EF Core on ASP.NET Core with an Existing Database

Important

The .NET Core SDK no longer supports project.json or Visual Studio 2015. Everyone doing .NET Core development is encouraged to migrate from project.json to csproj and Visual Studio 2017.

In this walkthrough, you will build an ASP.NET Core MVC application that performs basic data access using Entity Framework. You will use reverse engineering to create an Entity Framework model based on an existing database.

Tip

You can view this article's sample on GitHub.

Prerequisites

The following prerequisites are needed to complete this walkthrough:

Blogging database

This tutorial uses a Blogging database on your LocalDb instance as the existing database.

Tip

If you have already created the Blogging database as part of another tutorial, you can skip these steps.

  • Open Visual Studio
  • Tools -> Connect to Database...
  • Select Microsoft SQL Server and click Continue
  • Enter (localdb)\mssqllocaldb as the Server Name
  • Enter master as the Database Name and click OK
  • The master database is now displayed under Data Connections in Server Explorer
  • Right-click on the database in Server Explorer and select New Query
  • Copy the script, listed below, into the query editor
  • Right-click on the query editor and select Execute
CREATE DATABASE [Blogging];
GO

USE [Blogging];
GO

CREATE TABLE [Blog] (
    [BlogId] int NOT NULL IDENTITY,
    [Url] nvarchar(max) NOT NULL,
    CONSTRAINT [PK_Blog] PRIMARY KEY ([BlogId])
);
GO

CREATE TABLE [Post] (
    [PostId] int NOT NULL IDENTITY,
    [BlogId] int NOT NULL,
    [Content] nvarchar(max),
    [Title] nvarchar(max),
    CONSTRAINT [PK_Post] PRIMARY KEY ([PostId]),
    CONSTRAINT [FK_Post_Blog_BlogId] FOREIGN KEY ([BlogId]) REFERENCES [Blog] ([BlogId]) ON DELETE CASCADE
);
GO

INSERT INTO [Blog] (Url) VALUES
('http://blogs.msdn.com/dotnet'),
('http://blogs.msdn.com/webdev'),
('http://blogs.msdn.com/visualstudio')
GO

Create a new project

  • Open Visual Studio 2017
  • File -> New -> Project...
  • From the left menu select Installed -> Templates -> Visual C# -> Web
  • Select the ASP.NET Core Web Application (.NET Core) project template
  • Enter EFGetStarted.AspNetCore.ExistingDb as the name and click OK
  • Wait for the New ASP.NET Core Web Application dialog to appear
  • Under ASP.NET Core Templates 2.0 select the Web Application (Model-View-Controller)
  • Ensure that Authentication is set to No Authentication
  • Click OK

Install Entity Framework

To use EF Core, install the package for the database provider(s) you want to target. This walkthrough uses SQL Server. For a list of available providers see Database Providers.

  • Tools > NuGet Package Manager > Package Manager Console

  • Run Install-Package Microsoft.EntityFrameworkCore.SqlServer

We will be using some Entity Framework Tools to create a model from the database. So we will install the tools package as well:

  • Run Install-Package Microsoft.EntityFrameworkCore.Tools

We will be using some ASP.NET Core Scaffolding tools to create controllers and views later on. So we will install this design package as well:

  • Run Install-Package Microsoft.VisualStudio.Web.CodeGeneration.Design

Reverse engineer your model

Now it's time to create the EF model based on your existing database.

  • Tools –> NuGet Package Manager –> Package Manager Console
  • Run the following command to create a model from the existing database. If you receive an error stating The term 'Scaffold-DbContext' is not recognized as the name of a cmdlet, then close and reopen Visual Studio.
Scaffold-DbContext "Server=(localdb)\mssqllocaldb;Database=Blogging;Trusted_Connection=True;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models

Tip

You can specify which tables you want to generate entities for by adding the -Tables argument to the command above. E.g. -Tables Blog,Post.

The reverse engineer process created entity classes (Blog.cs & Post.cs) and a derived context (BloggingContext.cs) based on the schema of the existing database.

The entity classes are simple C# objects that represent the data you will be querying and saving.

using System;
using System.Collections.Generic;

namespace EFGetStarted.AspNetCore.ExistingDb.Models
{
    public partial class Blog
    {
        public Blog()
        {
            Post = new HashSet<Post>();
        }

        public int BlogId { get; set; }
        public string Url { get; set; }

        public virtual ICollection<Post> Post { get; set; }
    }
}

The context represents a session with the database and allows you to query and save instances of the entity classes.

public partial class BloggingContext : DbContext
{
   public virtual DbSet<Blog> Blog { get; set; }
   public virtual DbSet<Post> Post { get; set; }

   protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
   {
       if (!optionsBuilder.IsConfigured)
       {
           #warning To protect potentially sensitive information in your connection string, you should move it out of source code. See http://go.microsoft.com/fwlink/?LinkId=723263 for guidance on storing connection strings.
           optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Blogging;Trusted_Connection=True;");
       }
   }

   protected override void OnModelCreating(ModelBuilder modelBuilder)
   {
       modelBuilder.Entity<Blog>(entity =>
       {
           entity.Property(e => e.Url).IsRequired();
       });

       modelBuilder.Entity<Post>(entity =>
       {
           entity.HasOne(d => d.Blog)
               .WithMany(p => p.Post)
               .HasForeignKey(d => d.BlogId);
       });
   }
}

Register your context with dependency injection

The concept of dependency injection is central to ASP.NET Core. Services (such as BloggingContext) are registered with dependency injection during application startup. Components that require these services (such as your MVC controllers) are then provided these services via constructor parameters or properties. For more information on dependency injection see the Dependency Injection article on the ASP.NET site.

Remove inline context configuration

In ASP.NET Core, configuration is generally performed in Startup.cs. To conform to this pattern, we will move configuration of the database provider to Startup.cs.

  • Open Models\BloggingContext.cs
  • Delete the OnConfiguring(...) method
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    #warning To protect potentially sensitive information in your connection string, you should move it out of source code. See http://go.microsoft.com/fwlink/?LinkId=723263 for guidance on storing connection strings.
    optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Blogging;Trusted_Connection=True;");
}
  • Add the following constructor, which will allow configuration to be passed into the context by dependency injection
public BloggingContext(DbContextOptions<BloggingContext> options)
    : base(options)
{ }

Register and configure your context in Startup.cs

In order for our MVC controllers to make use of BloggingContext we are going to register it as a service.

  • Open Startup.cs
  • Add the following using statements at the start of the file
using EFGetStarted.AspNetCore.ExistingDb.Models;
using Microsoft.EntityFrameworkCore;

Now we can use the AddDbContext(...) method to register it as a service.

  • Locate the ConfigureServices(...) method
  • Add the following code to register the context as a service
// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    var connection = @"Server=(localdb)\mssqllocaldb;Database=Blogging;Trusted_Connection=True;";
    services.AddDbContext<BloggingContext>(options => options.UseSqlServer(connection));
}

Tip

In a real application you would typically put the connection string in a configuration file. For the sake of simplicity, we are defining it in code. For more information, see Connection Strings.

Create a controller

Next, we'll enable scaffolding in our project.

  • Right-click on the Controllers folder in Solution Explorer and select Add -> Controller...
  • Select Full Dependencies and click Add
  • You can ignore the instructions in the ScaffoldingReadMe.txt file that opens

Now that scaffolding is enabled, we can scaffold a controller for the Blog entity.

  • Right-click on the Controllers folder in Solution Explorer and select Add -> Controller...
  • Select MVC Controller with views, using Entity Framework and click Ok
  • Set Model class to Blog and Data context class to BloggingContext
  • Click Add

Run the application

You can now run the application to see it in action.

  • Debug -> Start Without Debugging
  • The application will build and open in a web browser
  • Navigate to /Blogs
  • Click Create New
  • Enter a Url for the new blog and click Create

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