实体框架6中关系从1到多到1到0或0.1的多重关系变化

Question

我与两个实体县和病人有一个1到多的关系。

public class County
{
   public int CountyId { get; set; }  // Primary Key
   public string CountyName { get; set; ) // A unique index column
   public virtual ICollection<Patient> Patients { get; set; }
}

public class CountyMap : EntityTypeConfiguration<County>
{
    public CountyMap()
    {
        ToTable("Counties");
        HasKey(c => c.CountyId);
        Property(c => c.CountyId).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
            Property(c => c.CountyName).IsRequired().HasMaxLength(50).HasColumnAnnotation("Index",
                new IndexAnnotation(new IndexAttribute("IX_Counties", 1) { IsUnique = true }));
    }
}

public class Patient
{
    public int PatientId { get; set; }
    public string PatientLastName { get; set; }
    public string PatientFirstName { get; set; }
    public string CountyName { get; set; }
    public int CountyId { get; set; } // Foreign key to Counties table
    public virtual County County { get; set; } // Navigation property
}

public class PatientMap: EntityTypeConfiguration<Patient>
{
    public PatientMap()
    {
            ToTable("Patients");
            HasKey(p => p.PatientId);
            Property(p => p.PatientId).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
            Property(p => p.PatientLastName).IsRequired().HasMaxLength(50);
            Property(p => p.PatientFirstName).IsRequired().HasMaxLength(50);
            Property(p => p.CountyId).IsRequired();
            HasRequired(p => p.County).WithMany(c => c.Patients).HasForeignKey(p => p.CountyId);
    }
}

public class AppContext : DbContext
{
    public AppContext()
        : base("name=AppContext")
    {
    }

    public virtual DbSet<County> Counties { get; set; }
    public virtual DbSet<Patient> Patients { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new CountyMap());
        modelBuilder.Configurations.Add(new PatientMap());
    }
}

public class PatientUOW
{
    public Patient CreatePatient(Patient patient)
    {
        string errorMessage = String.Empty;
        Patient patientReturned = null;
        County county = null;

        try
        {
           using (AppContext ctx = new AppContext())
           {
               // ONLY Pre-existing counties are permitted
                county = ctx.Counties.Where(c => c.CountyName == patient.CountyName).SingleOrDefault<County>();

                county.Patients.Add(patient);
                ctx.SaveChanges();  // An exception is thrown here

           }
        }
        catch (Exception err)
        {
        }
    }
}

例外消息是：

违反了多重性约束。关系'ArielOperations.Domain.Concrete.Patient_County‘的角色“Patient_County_Target”具有多重性1或0..1。

县实体上的调试器显示：

​

​

和

​

有人能解释一下这里发生了什么吗？我在这里和其他地方见过几个条目，但似乎都没有用。

谢谢。

Answer 1

我解决了我的问题。关键似乎是避免试图创建一个新的县记录时，增加一个病人在县。为了实现这一点，我没有将县实例传递给CreatePatient方法。新病人只包含目标县的CountyId。

Patient newPatient = new Patient
{
    PatientLastName = "Doe",
    PatientFirstName = "Jane",
    CountyName = "Denton",
    CountyId = 4
};

现在我们可以将这个newPatient实例传递给CreatePatient方法。

public Patient CreatePatient(Patient patient)
{
    string errorMessage = String.Empty;
    Patient patientReturned = null;
    County county = null;

    try
    {
            using (AppContext ctx = new AppContext())
           {
               // ONLY Pre-existing counties are permitted
                county = ctx.Counties.Where(c => c.CountyName == patient.CountyName).SingleOrDefault<County>();
                ctx.Patients.Add(patient);
                ctx.SaveChanges();
                patientReturned = patient;
           }
    } // end try
    catch (Exception err)
    {
        errorMessage = err.Message;
    } // end catch (Exception err)

    return patientReturned;
} // end public Patient CreatePatient(Patient patient)

这似乎有效，甚至通过外键将病人与县记录连接起来。似乎EF在这里做了一些事情来达到预期的目的。