我应该把 Database.EnsureCreated 放在哪里? [英] Where should I put Database.EnsureCreated?

问题描述

我有一个 Entity Framework Core + ASP.NET Core 应用程序，当我的应用程序启动时，我想确保创建了数据库，并且最终(一旦我进行了迁移)我想确保它们也运行.

I have an Entity Framework Core + ASP.NET Core application and when my application starts up I want to ensure that the database is created, and eventually (once I have migrations) I want to ensure that those are also run.

最初我将 Database.EnsureCreated() 放入我的 DbContext 的构造函数中，但是自从 DbContext 的一个新实例以来，每次有人点击我的应用程序时它似乎都会运行>DbContext 每次都会创建.

Initially I put Database.EnsureCreated() into the constructor of my DbContext but that appears to run every time someone hits my application since a new instance of the DbContext is created each time.

我试图将它放入我的启动代码中，但我需要一个 DbContext 的实例来执行此操作，目前尚不清楚如何获得该实例.我正在配置 EF:

I tried to put it into my startup code, but I need an instance of my DbContext to do that and it is unclear how exactly to get one. I am configuring EF as so:

serviceCollection.AddEntityFramework()
    .AddSqlServer()
    .AddDbContext<Models.MyContext>(options => options.UseSqlServer(...));

我没有看到从服务集合中获取 DbContext 实例的方法，也没有看到任何合适的单例可以将 DbContext 注入其中以便我可以进行一些一次性初始化.

I don't see a way to get an instance of the DbContext from the service collection, and I don't see any appropriate singleton to inject a DbContext into so I can do some one-time initialization.

那么，确保在每次应用程序运行时调用一次与我的 DbContext 相关的代码的最佳位置是什么?

So what is the best place to ensure some code related to my DbContext is called once per application run?

推荐答案

在撰写本文时，还没有一个正确"的地方可以在应用程序启动时运行代码以使其在请求范围内执行(请参阅 https://github.com/aspnet/Hosting/issues/373).

At the time of this writing, there is not a "correct" place to run code on application startup such that it executes within the request scope (see https://github.com/aspnet/Hosting/issues/373).

目前，解决方法是执行以下操作，但不适用于更复杂的多应用场景(参见 https://github.com/aspnet/EntityFramework/issues/3070#issuecomment-142752126)

For now, the workaround is to do the following, but it won't work in more complex multi-application scenarios (see https://github.com/aspnet/EntityFramework/issues/3070#issuecomment-142752126)

public class Startup
{
    ...

    public void Configure(IApplicationBuilder applicationBuilder, ...)
    {
        ...
        // NOTE: this must go at the end of Configure
        var serviceScopeFactory = applicationBuilder.ApplicationServices.GetRequiredService<IServiceScopeFactory>()
        using (var serviceScope = serviceScopeFactory.CreateScope())
        {
            var dbContext = serviceScope.ServiceProvider.GetService<MyDbContext>();
            dbContext.Database.EnsureCreated();
        }
    }
}

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