EF Core 8：如何为反序列化列表分配已有实体引用

我正在使用ASP.NET Core 8和EF Core 8构建Blazor Wasm Web应用，保存关联实体时触发了如下错误，寻求解决方法。

实体模型配置 #

public class EventRole
{
    // 复合主键
    public long EventId { get; set; }
    public string Name { get; set; }
    
    // 反向引用
    [ForeignKey(nameof(EventId))]
    public Event? Event { get; set; }
}

public class EventParticipant
{
    // 复合主键
    public long EventId { get; set; }
    public long UserId { get; set; }

    // 关联内容
    public ICollection<EventRole> Roles { get; set; } = new HashSet<EventRole>();

    [ForeignKey(nameof(EventId))]
    public Event? Event { get; set; }
    [ForeignKey(nameof(UserId))]
    public User? User { get; set; }
}

public class Event
{
    public long Id { get; set; }
    
    public ICollection<EventParticipant> Participants { get; set; } = new HashSet<EventParticipant>();
    public ICollection<EventRole> Roles { get; set; } = new HashSet<EventRole>();
}

DbContext模型构建配置 #

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    // 配置复合主键
    modelBuilder.Entity<EventParticipant>()
                .HasKey(p => new { p.EventId, p.UserId });
    modelBuilder.Entity<EventRole>()
                .HasKey(r => new { r.EventId, r.Name });

    // 注释掉的中间实体配置
    // modelBuilder.Entity<EventParticipantRole>().HasKey(pr => new { pr.EventId, pr.UserId, pr.RoleName });

    // 配置导航属性自动加载
    modelBuilder.Entity<Event>()
                .Navigation(e => e.Participants).AutoInclude();
    modelBuilder.Entity<Event>()
                .Navigation(e => e.Roles).AutoInclude();

    modelBuilder.Entity<EventParticipant>()
                .Navigation(p => p.User).AutoInclude();
    modelBuilder.Entity<EventParticipant>()
                .Navigation(p => p.Roles).AutoInclude();
}

Blazor客户端提交代码 #

Event ev = new Event();

EventParticipant participant = new EventParticipant()
{
    User = user
};

EventRole role = new EventRole
{
    Name = "Organisation"
};

EventRole role2 = new EventRole
{
    Name = "BudgetControl"
};

participant.Roles.Add(role);
participant.Roles.Add(role2);
ev.Roles.Add(role);
ev.Roles.Add(role2);
ev.Participants.Add(participant);

await Http.PostAsJsonAsync<Event>("api/Events", ev);

API控制器的保存方法 #

_context.Events.Add(@event);
await _context.SaveChangesAsync();

return CreatedAtAction(nameof(GetEvent), new { id = @event.Id }, @event);

触发的错误信息 #

System.InvalidOperationException: The value of 'EventRole.EventId' is unknown when attempting to save changes. This is because the property is also part of a foreign key for which the principal entity in the relationship is not known.

问题分析与疑问 #

我判断错误根源是JSON反序列化：客户端提交时同一个EventRole实例同时被添加到Event.Roles和EventParticipant.Roles，但反序列化后EF Core无法识别这两处的角色是同一实体，导致EventId无法被正确赋值。

请问如何让EF Core识别这些反序列化后的角色是事件中已有的实体，从而为其分配相同的实体引用（及对应的EventId）？是否需要修改当前的数据模型？注：当前使用内存数据库进行测试。

内容的提问来源于stack exchange，提问作者Apahdos