如何在Entity Framework ASP.NET MVC中的另一个实体内部的集合中添加一个实体

Question

I have two entities ( student , yogaspaceevent ). Yogaspaceevent which is like a exercise class, has a collection of students as RegisteredStudents and I want to be able to add students to this event to register them.

I tried a couple of things but can't seem to add a student to a yogaspaceevent, so maybe I'm not doing it correctly.

Question: do I need to create a whole new entity (say registeredstudents ) to add students to a yogaspaceevent ? Because I thought Entity Framework would somehow create this table for me or do some other magic under the hood to add a student to a event? The only solution I have is to create a RegisteredStudent entity and then EF will create a table. Is this the right solution?

Here are my two entities:

public class YogaSpaceEvent
{
    public int YogaSpaceEventId { get; set; }

    [Index] 
    public DateTime DateTimeScheduled { get; set; }
    public int AppointmentLength { get; set; }
    public int StatusEnum { get; set; }

    public virtual ICollection<Student.Student> RegisteredStudents { get; set; }   

    [Index]
    public int YogaSpaceRefId { get; set; }

    [ForeignKey("YogaSpaceRefId")]
    public virtual YogaSpace YogaSpace { get; set; }
}

public class Student
{
    public int StudentId { get; set; }
    public virtual StudentImage StudentImage { get; set; } //one-to-one
    [MaxLength(50)]
    public string CatchPhrase { get; set; }
    [MaxLength(250)]
    public string Education { get; set; } //Serra High School, San Diego State Univesity

    public string Work { get; set; } // Taco Bell, Starbucks
    [MaxLength(250)]
    public string WhyIPractice { get; set; }
    public StudentStatus Status { get; set; }
}

Here is my controller where I want to add a student to an event but I get exceptions thrown.

[HttpPost]
public ActionResult RegisterStudentForEvent(int eventId)
{
        try
        {
            Student student = _studentRepository.Find(User.Identity.GetUserId()); //User.Identity.GetUserId()
            if (student == null)
            {
                return Json( new { error = "notification", message = "You need an active student profile to register, please complete your student profile." });
            }

            var spaceEvent = _yogaSpaceEventRepository.Find(eventId);
            if (spaceEvent.RegisteredStudents == null)
            {
                spaceEvent.RegisteredStudents = new Collection<Student>(); // new EntityCollection<Student>(); neither of these two work, both throw exceptions
            }
            spaceEvent.RegisteredStudents.Add(student);

            _yogaSpaceEventRepository.Modify(spaceEvent);
            _yogaSpaceEventRepository.Save();

            // send email confirmation with a cancel link in it.

            return Json(new { error = "false", message = "Now registered!. A confirmation email has been sent. Have fun! " });
        }
        catch (Exception ex)
        {
            return Json( new { error = "true", message = ex.Message });
        }
    }

Here are the two exceptions that are thrown when I try both methods above.

The object could not be added to the EntityCollection or EntityReference. An object that is attached to an ObjectContext cannot be added to an EntityCollection or EntityReference that is not associated with a source object.

and

The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects.

Answer 1

I was having similar issue.

Your above code seems to be correct , I think you have missed to specify the mapping in dbcontext.cs class which resulted in below error:

The object could not be added to the EntityCollection or EntityReference. An object that is attached to an ObjectContext cannot be added to an EntityCollection or EntityReference that is not associated with a source object.

I hope my below example could help you.

---

Specialization (Parent Entity Class)

 [Table("M_Specialization")]
    public partial class Specialization : Entity
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        [Key]
        public int SID { get; set; }
        public string SCode { get; set; }
        public string Description { get; set; }
        public virtual ICollection<SubSpecialization> SubSpecializationDetails { get; set; }
}

---

Sub specialization (Child Entity Class)

[Table("M_SubSpecialization")]
    public partial class SubSpecialization : Entity
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        [Key]
        public Int32 SuID { get; set; }
        public string SubCode { get; set; }
        public Int32 SID { get; set; }  // This is required in order to map between specialization and subspecialization

        [ForeignKey("SID")]
        public virtual Specialization Specialization { get; set; }
    }

---

dbcontext.cs:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<SubSpecialization>().HasRequired(c => c.Specialization)
      .WithMany(s => s.SubSpecializationDetails)
      .HasForeignKey(c => c.SID);
}

Answer 2

You need to define a many-to-many relationship between Student and YogaSpaceEvent. The code you have created so far represents a one-to-many relationship.

An description of how to do that can be found here and here

What you need to add to your code is: - Add a ICollection property to your Student class - In your dbcontext.OnModelCreating, add code to define the many-to-many relationship.

As far as I can see, your controller function is OK.