How to make lazy-loading work with EF Core 2.1.0 and proxies

问题

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

回答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.

回答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

回答3:

I solved this by setting JSON serialization in service like this answer https://stackoverflow.com/a/49350457/4178475

来源：https://stackoverflow.com/questions/50180326/how-to-make-lazy-loading-work-with-ef-core-2-1-0-and-proxies