“The relationship could not be changed because one or more of the foreign-key properties is non-nullable…”

Question

Getting the following error upon save of my context:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

The controller code:

#region CreateStory
[HttpPost]
[Authorize]
public ActionResult CreateStory(Userstory story)
{
    if (ModelState.IsValid)
    {
        string _Username = User.Identity.Name.ToLower();

        using (var _Db = new NimbleContext())
        {
            Project _Project = _Db.Projects.First(p => p.Owner.ToLower() == _Username);

            if (_Project != null)
            {
                ProjectGroup _Group = _Db.Groups.First(g => g.GroupID == story.GroupID);

                if (_Group != null)
                {                            
                    _Db.Userstories.Add(story);
                    _Db.SaveChanges(); <--- Error Occurs here!
                    return Json(new { Success = true, GroupID = _Group.GroupID });
                }
            }                                            
        }
    }
    return Json(new { Success = false });
}
#endregion

The Model structure:

Project 1 -> * Groups 1 -> * Userstories

I can prevent the error from occurring by removing the following declarations:

Project _Project = _Db.Projects.First(p => p.Owner.ToLower() == _Username);
ProjectGroup _Group = _Db.Groups.First(g => g.GroupID == story.GroupID);

However I need to make these checks to ensure that the user belongs to the project etc. I really don't see what is causing the error, none of those statements should have an impact on the data to be saved right?

I am creating some tests, and I don't think I want to implement the workaround suggested by the answer. What did you end up doing? — Arturo Hernandez

Komengem Komengem · Accepted Answer · 2013-05-14T20:50:40

Its all in your model design. If you are explicitly declaring your Foreign Key properties, do them like this:

public class Project {
   [Key]
   [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
   public int ProjectId { get; set; }

   public virtual ICollection<Group> Groups { get; set; }
} 

public class Group {
   [Key]
   [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
   public int GroupId { get; set; }

   [ForeignKey("Project"), DatabaseGenerated(DatabaseGeneratedOption.None)]
   public int? ProjectId { get; set; }

   public virtual Project Project { get; set; }
   public virtual ICollection<Userstory> Userstories{ get; set; }
} 

public class Userstory {
   [Key]
   [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
   public int UserstoryId{ get; set; }

   [ForeignKey("Group"), DatabaseGenerated(DatabaseGeneratedOption.None)]
   public int? GroupId { get; set; }

   public virtual Group Group { get; set; }   
}

Note: This should work if you are following Entity Framework Code First, not sure about other methodologies like Model First and Database First

“The relationship could not be changed because one or more of the foreign-key properties is non-nullable…”

1 Answers