MVC 6 EF7 RC1 创建多个 dbcontext

Question

我正在尝试弄清楚如何在 EF7 RC1 中创建第二个数据库上下文。过去我可以使用带有 :base("connectionName") 的构造函数，但现在似乎不再是一个选项，因为它说不能将字符串转换为 System.IServiceProvider。

我的第二个上下文代码如下：

public class DecAppContext : DbContext
    {

        public DecAppContext()
          //  :base("DefaultConnection")
        {

        }
        public DbSet<VignetteModels> VignetteModels { get; set; }
        public DbSet<VignetteResult> Result { get; set; }
    }
}

在我的 config.json 中指定了连接：

"Data": {
    "DefaultConnection": {
      "ConnectionString": "Server=(localdb)\mssqllocaldb;Database=aspnet5-xxxxx...;Trusted_Connection=True;MultipleActiveResultSets=true"
    }
  }

在我启动的配置服务部分，我添加了两个上下文：

services.AddEntityFramework()
                .AddSqlServer()
                .AddDbContext<ApplicationDbContext>(options =>
                    options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]))
                .AddDbContext<DecAppContext>(options => options.UseSqlServer(Configuration["Data:DefaultConnection:ConnectionString"]));

applicationDB 上下文工作正常，因为我可以毫无问题地创建用户和登录

然而，当我尝试通过以下方式访问我的控制器中的其他上下文时：

private DecAppContext db = new DecAppContext();
var vignette = db.VignetteModels.SingleOrDefault(v => v.CaseId == vid);

我收到错误：

No database providers are configured. Configure a database provider by overriding OnConfiguring in your DbContext class or in the AddDbContext method when setting up services.

EF7 RC1 中具有多个数据库上下文并访问它们的任何工作示例将不胜感激。

Answer 1

首先，我会向您推荐 GitHub 上 EntityFramework 的 wiki the article。文章介绍了很多定义DbContext的方法，其中引用了appsettings.json的一段。我个人更喜欢使用 [FromServices] 属性的方式。

代码可能如下：

首先你定义了appsettings.json，内容如下

{
  "Data": {
    "ApplicationDbConnectionString": "Server=(localdb)\mssqllocaldb;Database=ApplicationDb;Trusted_Connection=True;MultipleActiveResultSets=true",
    "DecAppDbConnectionString": "Server=Server=(localdb)\mssqllocaldb;Database=DecAppDb;Trusted_Connection=True;MultipleActiveResultSets=true"
  }
}

在其中定义两个连接字符串。

你声明 classes DecAppContext 和 ApplicationDbContext 的秒数，它们以 DbContext 作为基础 class。最简单的形式就是

public class ApplicationDbContext : DbContext
{
}
public class DecAppContext : DbContext
{
}

没有任何 DbSet 属性。

第三步。您使用 Microsoft.Extensions.DependencyInjection 注入数据库上下文。为此，您只需在 Startup.cs 中包含

之类的内容

public class Startup
{
    // property for holding configuration
    public IConfigurationRoot Configuration { get; set; }

    public Startup(IHostingEnvironment env)
    {
        // Set up configuration sources.
        var builder = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json")
            .AddEnvironmentVariables();
        // save the configuration in Configuration property
        Configuration = builder.Build();
    }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        // Add framework services.
        services.AddMvc()
            .AddJsonOptions(options => {
                options.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
            });
        services.AddEntityFramework()
            .AddSqlServer()
            .AddDbContext<ApplicationDbContext>(options => {
                options.UseSqlServer(Configuration["Data:ApplicationDbConnectionString"]);
            })
            .AddDbContext<DecAppContext>(options => {
                options.UseSqlServer(Configuration["Data:DecAppDbConnectionString"]);
            });
    }
    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        ...
    }
}

Se 使用配置 "Data:DecAppDbConnectionString" 和 "Data:ApplicationDbConnectionString".

创建两个 DbContext（DecAppContext 和 ApplicationDbContext）

现在我们可以在控制器中使用上下文了。例如

[Route("api/[controller]")]
public class UsersController : Controller
{
    [FromServices]
    public ApplicationDbContext ApplicationDbContext { get; set; }

    [FromServices]
    public DecAppContext DecAppContext { get; set; }

    [HttpGet]
    public IEnumerable<object> Get() {
        var returnObject = new List<dynamic>();

        using (var cmd = ApplicationDbContext.Database.GetDbConnection().CreateCommand()) {
            cmd.CommandText = "SELECT Id, FirstName FROM dbo.Users";
            if (cmd.Connection.State != ConnectionState.Open)
                cmd.Connection.Open();

            var retObject = new List<dynamic>();
            using (var dataReader = cmd.ExecuteReader())
            {
                while (dataReader.Read())
                {
                    var dataRow = new ExpandoObject() as IDictionary<string, object>;
                    for (var iFiled = 0; iFiled < dataReader.FieldCount; iFiled++)
                        dataRow.Add(
                            dataReader.GetName(iFiled),
                            dataReader.IsDBNull(iFiled) ? null : dataReader[iFiled] // use null instead of {}
                        );

                    retObject.Add((ExpandoObject)dataRow);
                }
            }
            return retObject;
        }
    }
}

或相同使用async/await:

[Route("api/[controller]")]
public class UsersController : Controller
{
    [FromServices]
    public ApplicationDbContext ApplicationDbContext { get; set; }

    [FromServices]
    public DecAppContext DecAppContext { get; set; }

    [HttpGet]
    public async IEnumerable<object> Get() {
        var returnObject = new List<dynamic>();

        using (var cmd = ApplicationDbContext.Database.GetDbConnection().CreateCommand()) {
            cmd.CommandText = "SELECT Id, FirstName FROM dbo.Users";
            if (cmd.Connection.State != ConnectionState.Open)
                cmd.Connection.Open();

            var retObject = new List<dynamic>();
            using (var dataReader = await cmd.ExecuteReaderAsync())
            {
                while (await dataReader.ReadAsync())
                {
                    var dataRow = new ExpandoObject() as IDictionary<string, object>;
                    for (var iFiled = 0; iFiled < dataReader.FieldCount; iFiled++)
                        dataRow.Add(dataReader.GetName(iFiled), dataReader[iFiled]);

                    retObject.Add((ExpandoObject)dataRow);
                }
            }
            return retObject;
        }
    }
}

可以只用 [FromServices] 属性声明属性 public ApplicationDbContext ApplicationDbContext { get; set; }，然后 ASP.NET 从 ConfigureServices 中注入的上下文对其进行初始化。以同样的方式，您可以在需要时使用第二个上下文 DecAppContext。

上面的代码示例将在数据库上下文中执行SELECT Id, FirstName From dbo.Users 和return JSON 形式的数据[{"id":123, "firstName":"Oleg"},{"id":456, "firstName":"Xaxum"}]。由于 AddJsonOptions 在 ConfigureServices.

更新： 我必须参考 the announcement. The next version of MVC (RC2) will require to change the above code to use [FromServices] as additional parameter (of method Get() for example) instead of usage public property [FromServices] public ApplicationDbContext ApplicationDbContext { get; set; }. One will need to remove the property ApplicationDbContext and to add additional parameter to Get() method: public async IEnumerable<object> Get([FromServices] ApplicationDbContext applicationDbContext) {...}. Such changes can be easy done. See here 和 MVC 演示示例中的更改示例：

[Route("api/[controller]")]
public class UsersController : Controller
{
    [HttpGet]
    public async IEnumerable<object> Get(
                     [FromServices] ApplicationDbContext applicationDbContext,
                     [FromServices] DecAppContext decAppContext)
    {
        var returnObject = new List<dynamic>();

        // ...  the same code as before, but using applicationDbContext 
        // and decAppContext parameters instead of ApplicationDbContext
        // and DecAppContext properties
    }

MVC 6 EF7 RC1 创建多个 dbcontext

MVC 6 EF7 RC1 creating multiple dbcontexts

entity-framework-core

asp.net-core-mvc