实体框架，Pk，Fk，有时会出错

Question

I get sometimes this error, when I want to insert something:

The INSERT statement conflicted with the FOREIGN KEY constraint "FK_dbo.Climb_dbo.DifficultGrade_DifficultGradeID". The conflict occurred in database "ContosoUniversity2", table "dbo.DifficultGrade", column 'DifficultGradeID'.

The statement has been terminated.

But sometimes, I get this error, and sometimes I dont get this error

I have this insert:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(ClimbViewModel ModelViewClimb)
{
    try
    {
        if (ModelState.IsValid)
        {
            var climb = new Climb();
            UpdateCLimb(climb, ModelViewClimb);
            db.Climbs.Add(climb);

            db.SaveChanges();
            return RedirectToAction("Index");
        }
    }
    catch (RetryLimitExceededException /* dex */)
    {
        ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists, see your system administrator.");
    }

    ViewBag.Id = new SelectList(db.countries, "Id", "country_name", ModelViewClimb.Id);
    ViewBag.DifficultGradeID = new SelectList(db.DifficultGrades, "DifficultGradeID", "NameDifficult", ModelViewClimb.Id);
    return View(new ClimbViewModel() );
}

These are my models:

public class Climb
{
    // public Climb climb { get; set; }

    [Key]
    public int climbID { get; set; }
    //[Display(Name="difficulty")]
    public string Name { get; set; }

    public int? UserProfileID { get; set; }
    public int? CountryID { get; set; }
    public int? DifficultGradeID { get; set; }

    public virtual UserProfile userProfile { get; set; }
    public virtual Country country { get; set; }
    public virtual DifficultGrade difficult { get; set; }
}

and

public class DifficultGrade
{
    [Key]
    public int DifficultGradeID { get; set; }
    [Display(Name = "difficulty")]
    public string NameDifficult { get; set; }
    public virtual ICollection<Climb> Climbs { get; set; }
    public virtual ICollection<Route> Routes { get; set; }
}

I also tell what is has to be, a user can just name a climb and select a country and a difficult level (easy, normal, hard) so two dropdownlists and one textbox field. By adding a new climb, and I see that sometimes climbId and DifficultGradeID have the same id.

Answer 1

Please, for questions like this, always post your models and the code for any custom methods you're using, ie UpdateClimb . It's next to impossible to give you any good guidance without such information.

However, in this case, I can make an assumption about what your problem is based on the error. Most likely, UpdateClimb , among other things, set's a many-to-many navigation property on Climb with a list directly. For example:

climb.SomeM2MNavigationProperty = someEnumerable;

If any items in that enumerable already exists on the relationship, you'll get this error as when you replace the entire list, Entity Framework treats each item as a newly added relationship. When it inserts this new, duplicate record, the database screams.

What you should do instead in situations like this is selectively add and remove items form the existing relationships.

// Remove items that are no longer related
entity.SomeM2MNavigationProperty.Where(m => !newRelationshipItemIds.Contains(m.Id))
    .ToList().ForEach(m => entity.SomeM2MNavigationProperty.Remove(m));

// Then you need to add any new relationships
var existingRelationshipItemIds = entity.SomeM2MNavigationProperty.Select(m => m.Id).ToList();
db.EntityDbSet.Where(m => newRelationshipItemIds.Except(existingRelationshipItemIds).Contains(m.Id))
    .ToList().ForEach(m => entity.SomeM2MNavigationProperty.Add(m));