实体框架6中关系的多样性从1对多变为1对0或0..1

Question

I am working with two entities County and Patient which have a 1 to many relationship.

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)
        {
        }
    }
}

The exception message is:

Multiplicity constraint violated. The role 'Patient_County_Target' of the relationship 'ArielOperations.Domain.Concrete.Patient_County' has multiplicity 1 or 0..1.

The debugger on the County entity shows:

and

Can anyone explain what is happening here? I've seen several entries here and elsewhere and none seem to work.

Thank you.

Answer 1

I was able to resolve my issue. The key seems to be to avoid attempting to create a new County record when adding a Patient in a County. To achieve this I did not pass a County instance to the CreatePatient method. The new Patient only contains the CountyId of the target County.

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

We can now pass this newPatient instance to the CreatePatient method.

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)

This seems to work and even connect the Patient to the County record via the foreign key. It seems EF does some things here to achieve the desired end.