模型使用双向导航属性多次同一实体

Question

测量和观察之间存在一对一的关系，但观察结果有＆＃34;之前＆＃34; ＆＃34;＆＃34;＆＃34;＆＃34;测量。只要我不需要导航属性从测量返回到观察，这就可以正常工作。实体无法解析Measurement.observation属性，导致normalizedValue属性引发NullPointer异常。

编辑：已编辑以尝试包含来自How can I set up two navigation properties of the same type in Entity Framework的评论，但仍无效。

public class Measurement
{
    [Key]
    public int measurementId { get; set; }
    public int val1 { get; set; }

    public virtual Observation observation { get; set; }

    public float noramlizedValue { get { return val1 / observation.normalizationFactor; } }
}

public class Observation
{
    [Key]
    public int observationId { get; set; }

    [Required]
    public int beforeMeasId { get; set; }
    public virtual Measurement before { get; set; }

    [Required]
    public int afterMeasId { get; set; }
    public virtual Measurement after { get; set; }

    public int normalizationFactor { get; set; }
}

public class Model1 : DbContext
{

    public virtual DbSet<Observation> observations { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Observation>()
            .HasRequired(o => o.before)
            .WithMany()
            .HasForeignKey(o => o.beforeMeasId)
            .WillCascadeOnDelete(true);

        modelBuilder.Entity<Observation>()
            .HasRequired(o => o.after)
            .WithMany()
            .HasForeignKey(o => o.afterMeasId)
            .WillCascadeOnDelete(false);
    }
}

class Program
{
    static void Main(string[] args)
    {
        using (var db = new Model1())
        {
            db.Configuration.LazyLoadingEnabled = true;
            var o1 = new Observation
            {
                before = new Measurement { val1 = 5 },
                after = new Measurement { val1 = 10 },
                normalizationFactor = 3
            };
            db.observations.Add(o1);
            db.SaveChanges();

            Console.WriteLine("Before: ");
            Console.WriteLine(o1.before.noramlizedValue);  // Raises null reference exception
            Console.WriteLine("After: ");
            Console.WriteLine(o1.after.noramlizedValue);

        }
    }