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我試圖將一些新創建的實體添加到我的數據庫中，但是EF正在向消息中拋出一個InvalidOperationExceptionThe relationship could not be changed because one or more of the foreign-key properties is non-nullable但它不告訴我哪個關係是問題。EF添加實體：關係無法更改，因爲一個或多個外鍵屬性不可爲空
下面是相關實體：
DatabaseContext OnModelCreating：
...
modelBuilder.Entity<Student>()
.HasRequired(s => s.Home)
.WithMany(s => s.StudentsInResidence)
.HasForeignKey(s => s.HomeId);
modelBuilder.Entity<SignoutRequest>()
.HasOptional(x => x.Destination)
.WithMany(x => x.RequestsToHere)
.HasForeignKey(x => x.DestinationId);
modelBuilder.Entity<SignoutRequest>()
.HasRequired(x => x.Subject)
.WithMany(x => x.SignoutRequests)
.HasForeignKey(x => x.SubjectId);
...
SignoutRequest：
public class SignoutRequest
{
public SignoutRequest()
{
}
public int Id { get; set; }
public string SubjectId { get; set; }
...
public int? DestinationId { get; set; }
#region Custom Fields
...
#endregion
#region Virtual Properties
...
public virtual Student Subject { get; set; }
public virtual DeviceGroup Destination { get; set; }
#endregion
}
學生：
public class Student : IWpEntity
{
public string Id { get; set; }
public string Name { get; set; }
public string EmailAddress { get; set; }
public string PhoneNumber { get; set; }
public int Form { get; set; }
public int HomeId { get; set; }
public virtual DeviceGroup Home { get; set; }
public virtual ICollection<BlacklistEntry> BlacklistEntries { get; set; }
public virtual ICollection<SignoutRequest> SignoutRequests { get; set; }
}
DeviceGroup：
public class DeviceGroup : IWpEntity
{
public int Id { get; set; }
public string FriendlyName { get; set; }
public virtual ICollection<Device> Devices { get; set; }
public virtual ICollection<Student> StudentsInResidence { get; set; }
public virtual ICollection<SignoutRequest> RequestsToHere { get; set; }
}
IWpEntity只是一個方法集合。
這裏是拋出錯誤代碼：
_database.SubmittedRequests.Add(srq);
await _database.SaveChangesAsync();
srq的是，我試圖添加到數據庫中的SignoutRequest。它已將屬性SubjectId和DestinationId設置爲數據庫中已有對象的有效ID。