在 MVC5 ASP.Net 应用程序中使用 Repository 和 UnitOfWork 模式时，如何修改数据库初始值设定项 Seed 方法？

Question

I've had a look through the tutorial https://docs.microsoft.com/en-us/aspnet/mvc/overview/older-versions/getting-started-with-ef-5-using-mvc-4/implementing-the-repository-and-unit-of-work-patterns-in-an-asp-net-mvc-application

(I'm actually using MVC5 and even that's old now!! - The MVC5 version of the same tutorial didn't go into repository and unit of work much).

I've implemented the patterns as described, and replaced the code in my controllers, but am left puzzling over my Database Initializer that I created earlier in the tutorial.

It has a method

protected override void Seed( MyContext context )

It seems now that I ought to be changing the code here to use UnitOfWork too, rather than MyContext

I can't do that though, because this is an overridden method and I guess the method signature has to be just as it is to be called at the right time.

Further, the instance of MyContext that is in UnitOfWork is private, so even if I could pass a unit of work in I still couldn't use its context.

So the question is once you've done this and implemented Repository and UnitOfWork, how do you fix up this initialization code, or how else do you initialize your database with seed data?

Answer 1

So, I think this was a bit of n00b error on my part, and some of you that had looked at this probably wondered what on earth I was on about, but I'll leave this answer here just in case it helps anyone else.

Following the tutorial I'd created a

public class GenericRepository<TEntity> where TEntity : class
{
    internal PayloadContext context;
    internal DbSet<TEntity> dbSet;

    public GenericRepository( PayloadContext context )
    {
        this.context = context;
        this.dbSet = context.Set<TEntity>();
    }

    ....
}

And I hadn't really looked closely enough at that second line in the constructor.

I'd seen that the context was now created in th UnitOfWork, and passed into the various repositories so that all the repositories could share it.

But I'd thought, incorrectly, that what was happening above was the a DbSet was being created for this repository. Therefore I removed the declaration of the DbSet properties from my context, thinking I'd never use them, and that I'd now be accessing the data through the repositories.

In other words in the controllers, for example, I thought I was never doing:

MyContext db = new MyContext();

and

public ActionResult Index()
{
    return View( db.MySetPropertyDeclaredInMyContext.Get() )
}

but rather

private UnitOfWork unitOfWork = new UnitOfWork();

and

public ActionResult Index()
{
    return View( unitOfWork .MySetPropertyDeclaredInMyUoW.Get() )
}

By the time I got back to "fixing up" the initialisation code I was was stuck because that had the context passed in and the "old" entity sets directly referenced. Further I don't call the Seed method myself from my own code.

What I realised is that the second line in the constructor of the GenericRepository was only setting a reference to a DBSet (that's a method, not a 'new'). What's more "Entity Framework requires that this method return the same instance each time that it is called for a given context instance and entity type." With that in mind, I still needed my original DbSet property declarations inside the context.

The DatabaseInitializer Seed method could remain unchanged.

For completeness, for those wondering where this is called: An implementation of this interface [IDatabaseInitializer] is used to initialize the underlying database when an instance of a DbContext derived class is used for the first time. This initialization can conditionally create the database and/or seed it with data. The strategy used is set using the static InitializationStrategy property of the Database class

So in other words, this initialisation code was fine and would be triggered first time the context was used which would be first time a UnitOfWork was performed. (Or maybe even invoked).