如何使用EF Core 2.1.0和代理进行延迟加载

Question

I have the following models:

public class Session
{
    public int SessionID { get; set; }
    public int UserID { get; set; }

    public virtual User User { get; set; }
}

public class User
{
    public int UserID { get; set; }
    public int OrganizationID { get; set; }

    public virtual ICollection<Session> Sessions { get; set; }
    public virtual Organization Organization { get; set; }
}

public class Organization
{
    public int OrganizationID { get; set; }

    public virtual ICollection<User> Users { get; set; }
}

that are registered in DbContext as:

modelBuilder.Entity<Session>(entity =>
{
    entity.ToTable("sessions");

    entity.Property(e => e.SessionID).HasColumnName("id");
    entity.Property(e => e.UserID).HasColumnName("user_id");

    entity.HasOne(e => e.User)
        .WithMany(e => e.Sessions)
        .HasForeignKey(e => e.UserID);
}

modelBuilder.Entity<User>(entity =>
{
    entity.ToTable("users");

    entity.Property(e => e.UserID).HasColumnName("id");
    entity.Property(e => e.OrganizationID).HasColumnName("organization_id");

    entity.HasOne(e => e.Organization)
        .WithMany(e => e.Users)
        .HasForeignKey(e => e.OrganizationID);
}

modelBuilder.Entity<Organization>(entity =>
{
    entity.ToTable("organizations");

    entity.Property(e => e.OrganizationID).HasColumnName("id");
}

I'm trying to use lazy loading with Microsoft.EntityFrameworkCore.Proxies as described here :

builder.Register(c =>
{
    var optionsBuilder = new DbContextOptionsBuilder<Context>();
    optionsBuilder
        .UseLazyLoadingProxies()
        /* more options */
        ;

    var opts = optionsBuilder.Options;

    return new Context(opts);
}).As<DbContext>().InstancePerLifetimeScope();

I'm querying sessions using context.All<Session> . However, Session.User and Session.User.Organization are null by default. To load them I have to do something like context.All<Session>().Include(s => s.User).Include(s => s.User.Organization) . How can I avoid that? Why doesn't UseLazyLoadingProxies work?

---

• .NET Core version: 2.1.300-preview2-008533
• Target: netcoreapp2.1
• EF Core (and Proxies) version: 2.1.0-preview2-final

Answer 1

Steps To Configure Lazy Loading with Proxies in Asp.net Core 2.1

1. Install Microsoft.EntityFrameworkCore.Proxies package
2. Enable LazyLoadingProxies You can enable it with a call to UseLazyLoadingProxies:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    => optionsBuilder
        .UseLazyLoadingProxies()
        .UseSqlServer(myConnectionString);

Or when using AddDbContext:

.AddDbContext<BloggingContext>(
b => b.UseLazyLoadingProxies()
      .UseSqlServer(myConnectionString));

3. EF Core will then enable lazy loading for any navigation property that can be overridden--that is, it must be virtual.

Answer 2

You can try to configure proxies in Startup.cs like

public void ConfigureServices(IServiceCollection services)
{
    #region Database configuration

    // Database configuration
    services.AddDbContext<DbContext>(options =>
        options.UseLazyLoadingProxies()
            .UseSqlServer(Configuration.GetConnectionString("MyConnectionString")));

    #endregion Database configuration
}

And by the way you can already update you app packages to pure 2.1.0 (not final or RC). One of the reasons why your configuration may not work is an unstable version of components.

NB: Microsoft.EntityFrameworkCore.Proxies.dll is installed from nuget independently from EFCore

Answer 3

我通过在服务中设置JSON序列化来解决这个问题，就像这个答案https://stackoverflow.com/a/49350457/4178475