连接数据库

通过 Entity Framework Core 连接数据库，需要做一些准备工作。因为你通过 dotnet new 让项目使用了 MVC + Individual认证 项目模板，这些准备已经就绪了:

• Entity Framework Core包 这些都默认包括在了所有 ASP.NET Core 项目中。

• 数据库(必须地呀) 经由 dotnet new 指令，在项目的根目录下生成了小型的 SQLite 数据库文件 app.db。 SQLite 是一个轻量级数据库引擎，可以运行在你的机器上而不必安装任何额外工具，所以在开发环境下使用起来既方便又快捷。

• 数据库上下文 数据库上下文(database context)是一个 C# 类提供的数据库入口点。 你的代码就是通过它与数据库交互，进行读写的。Data/ApplicationDbContext.cs 文件里，就保存着一个很基本的数据库上下文。

• 连接字符串 无论你连接本地文件数据库(SQLite)还是位于其它主机的数据库，都需要定义一个字符串，其中包含 数据库的名字 或 用来连接的数据库地址。 这一项已经在 appsettings.json 文件里为你设置好了: SQLite 数据库的 连接字符串 是 DataSource=app.db。

Entity Framework Core 借助 数据库上下文、连接字符串 与数据库建立连接。你需要在 Startup 类里的 ConfigureServices 方法中为 Entity Framework Core 指定所用的 数据库上下文、连接字符串和数据库类型。感谢项目模板：

services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlite(
        Configuration.GetConnectionString("DefaultConnection")));

这段代码把 ApplicationDbContext 添加到服务容器里，并通过配置(appsettings.json)中的 连接字符串 指定 Entity Framework Core 使用 SQLite 数据库。

如你所见，dotnet new 为你完成了很多工作! 数据库已经配置好待用了。但是还没有表用于保存 待办事项条目。为了能存储 TodoItem 实体，你需要修改数据库上下文，并对数据库进行变更。

Connect to a database

There are a few things you need to use Entity Framework Core to connect to a database. Since you used dotnet new and the MVC + Individual Auth template to set your project, you’ve already got them:

• The Entity Framework Core packages. These are included by default in all ASP.NET Core projects.

• A database (naturally). The app.db file in the project root directory is a small SQLite database created for you by dotnet new. SQLite is a lightweight database engine that can run without requiring you to install any extra tools on your machine, so it’s easy and quick to use in development.

• A database context class. The database context is a C# class that provides an entry point into the database. It’s how your code will interact with the database to read and save items. A basic context class already exists in the Data/ApplicationDbContext.cs file.

• A connection string. Whether you are connecting to a local file database (like SQLite) or a database hosted elsewhere, you’ll define a string that contains the name or address of the database to connect to. This is already set up for you in the appsettings.json file: the connection string for the SQLite database is DataSource=app.db.

Entity Framework Core uses the database context, together with the connection string, to establish a connection to the database. You need to tell Entity Framework Core which context, connection string, and database provider to use in the ConfigureServices method of the Startup class. Here’s what’s defined for you, thanks to the template:

services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlite(
        Configuration.GetConnectionString("DefaultConnection")));

This code adds the ApplicationDbContext to the service container, and tells Entity Framework Core to use the SQLite database provider, with the connection string from configuration (appsettings.json).

As you can see, dotnet new creates a lot of stuff for you! The database is set up and ready to be used. However, it doesn’t have any tables for storing to-do items. In order to store your TodoItem entities, you’ll need to update the context and migrate the database.