Entity Framework 4 Code First是否支持像NHibernate这样的身份生成器？ [英] Does Entity Framework 4 Code First have support for identity generators like NHibernate?

问题描述

这个问题，一年前的问题是类似的：
Entity Framework 4是否支持像NHibernate这样的id值生成器？

This question, asked a year ago, is similar: Does the Entity Framework 4 support generators for id values like NHibernate?

但是我想知道什么如果代码第一个CTP增加了对身份生成策略的支持。如果没有，有没有人在EF中知道一个很好的扩展点来实现类似的东西？

But what I'd like to know is if the code first CTP adds support for identity generation strategies. If not, does anyone know a good extension point in EF to implement something similar?

我正在使用使用GUID作为标识符的模型类。当使用EF插入时，它们保留其 Guid.Empty 初始值。我知道您可以将DB中的列的默认值设置为 newid（），但这不符合客户端身份生成的目的。

I'm currently working with model classes which use GUID as the identifier. When inserting using EF they retain their Guid.Empty initial values. I know that you can set a default value for the column in the DB to newid() but that defeats the purpose of client-side identity generation.

实体框架是否还不够成熟，无法在分布式，断开连接的系统中使用？

Is Entity Framework just not mature enough to be used in a distributed, disconnected system?

推荐答案

不，实体框架代码优先仍然是EFv4周围的好包装。没有像发电机这样的NHibernate。如果您想要客户端Id生成器，您将必须在派生的 DbContext 中覆盖 SaveChanges ，并实现您自己的分配Ids的逻辑

No, Entity framework code-first is still just nice wrapper around EFv4. There are no NHibernate like generators. If you want client side Id generator you will have to override SaveChanges in derived DbContext and implement your own logic of assigning Ids to new entities.

编辑

某些高级示例： p>

Some high level example:

public class Context : DbContext
{
    // Helper for example
    // DO NOT USE IN REAL SCENARIOS!!!
    private static int i = 0; 

    public DbSet<MyEntity> MyEntities { get; private set; }

    public Context()
        : base("connection")
    {
        MyEntities = Set<MyEntity>();
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<MyEntity>().HasKey(e => e.Id);
        // Turn off autogeneration in database
        modelBuilder.Entity<MyEntity>()
                    .Property(e => e.Id)
                    .HasDatabaseGeneratedOption(HasDatabaseGeneratedOption.None);

        // Other mapping
    }

    public override int SaveChanges()
    {
        foreach (var entry in ChangeTracker.Entries<MyEntity>()
            .Where(e => e.State == EntityState.Added))
        {
            // Here you have to add some logic to generate Id
            // I'm using just static field
            entry.Entity.Id = ++i;  
        }

        return base.SaveChanges();
    }
}

public class MyEntity
{
    public int Id { get; set; }
    // Other properties
}

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