动态ConnString对DbContext的依赖注入

Question

Below is a simple but functional example of roughly how I would do Dependency Injection. This works great when my DbContext connection string is not dynamic. Even if it's passed in to the factory through a config file or whatever, it doesn't matter so long as it's the same one all the time.

What I need is to wrap my head around how to make (ideally minor) modifications to the below code to allow for the connection string to be determined dynamically at run time.

For example, say on the View the user was able to not only select the teacher to be passed into the Post method of the controller, but also the school. If, for simplicity sake, there are 2 schools that have the exact same database structure, but have different connection strings how do I get that down from the controller to the factory?

I've experimented with passing a value from method to method, but this isn't really sustainable for large projects, increases the likelihood of errors and overall is just messy (besides violations of SOLID) to be passing something from layer to layer like that. (If desired I can add the not exactly ideal attempts I've made, I've omitted them for brevity sake since this is already a fairly long question what with the code examples and all).

Controller

public class HomeController : Controller
{
    private readonly IDataService _dataService;

    public HomeController(IDataService dataService)
    {
        _dataService = dataService;
    }

    public ActionResult Index()
    {

        var results = _dataService.GetTeachers();
        var model = new ViewModel
        {
            Teachers = results
        };
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(ViewModel model)
    {
        var results = _dataService.GetCourses(model.Teacher);
        model.Courses = new List<string>(results);
        return View(model);
    }
}

Service

public class DataService : IDataService
{
    private readonly IDataRepo _dataRepo;

    public DataService(IDataRepo dataRepo)
    {
        _dataRepo = dataRepo;
    }

    public List<string> GetCourses(string teacherName)
    {
        return _dataRepo.GetCourses()
            .Where(c => c.Teacher.FirstName == teacherName)
            .Select(c => c.Name)
            .ToList();
    }

    public List<string> GetTeachers()
    {
        return _dataRepo.GetCourses()
            .Select(c => c.Teacher.FirstName)
            .ToList();
    }
}

Repository

public class DataRepo : IDataRepo
{
    private readonly SchoolContext _context;

    public DataRepo()
    {
        _context = ContextFactory.MakeContext();
    }

    public IEnumerable<Course> GetCourses()
    {
        return _context.Courses;
    }
}

Context Factory

public static class ContextFactory
{
    public static SchoolContext MakeContext()
    {
        var connString =
            "connStringA";
        return new SchoolContext(connString);
    }
}

UnityConfig

    public static void RegisterComponents()
    {
        var container = new UnityContainer();

        container.RegisterType<IDataService, DataService>();
        container.RegisterType<IDataRepo, DataRepo>();

        DependencyResolver.SetResolver(new UnityDependencyResolver(container));
    }

Answer 1

First, you have to decide how are you going to get the current connection string to use. Is it through a URL? or using the current user or whatever other way.

Then, create another database that has a mapping between the connection strings and the method you chose (user, url ...)

Lastly, implement a way to get the record from the database.

so, assuming that you will use the URL as an identifier for the current tenant, your entity class should be like this:

public class Tenant
{
   public string Url {get;set;}
   public string ConnectionString {get;set;}
}

An interface that represents the logic to get the current tenant:

public interface ICurrentTenantService
{
  Tenant GetCurrentTenant();
}

And now you will put its implementation

public class CurrentTenantService : ICurrentTenantService
{
  public Tenant GetCurrentTenant()
   {
      string currentUrl = HttpContext.Current.Url; //make sure to get only the base URL here
      return TenantDbContext.Tenants.FirstOrDefault(t=>t.Url == url); //TenantDbContext should be a class that has the Tenant entity
   }
}

Now you have to wire up the context factory to the tenant service like this

public static class ContextFactory
{
    private readonly ICurrentTenantService currentTenantService;
   //Inject it in the constructor

    public static SchoolContext MakeContext()
    {
        var currentTenant= currentTenantService.GetCurrentTenant(); //Check for NULL

        return new SchoolContext(currentTenant.ConnectionString);
    }
}