当IDENTITY_INSERT设置为OFF时，无法在TABLE中为Identity列插入显式值； .NET Core 2.1

Question

I'm trying to clone entities in EF Core. As far as I understand, I need to retrieve them AsNoTracking and then re-add them however when I do I get the error:

Cannot insert explicit value for identity column in Exercises when IDENTITY_INSERT is set to OFF;

I added the data annotation: [DatabaseGenerated(DatabaseGeneratedOption.Identity)] to the Id but I'm still getting the same error.

Here is my code:

public class Exercise : Entity
{
    public Exercise()
    {
        Sets = new HashSet<Set>();
    }       

    public DateTime Date { get; set; }

    public int MuscleGroupId {get;set;}

    public int MesoId { get; set; }

    public ICollection<Set> Sets { get; set; }

    public void AddSet(Set set)
    {
        Sets.Add(set);
    }
}

And here is my code for Entity which has I set an annotation telling EF Core the ID is database generated:

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

Here is the code to clone the entities:

 var exercises = _context.Exercises.AsNoTracking().Include(y => y.Sets)
.Where(x => x.MesoId == mesoId && x.Date == fromDate);

 foreach (var exercise in exercises)
   {
      exercise.Date = toDate;
      _context.Exercises.Add(exercise);
   }

 await _context.SaveChangesAsync();

Answer 1

You cannot store values into an Identity column in the database because that value is managed by the database itself.

When "cloning" your Exercise records you need to set Id=0 so that when the new row gets inserted the database will create the Identity value for it, eg:

foreach (var exercise in exercises)
{
   exercise.Date = toDate;
   exercise.Id = 0; //<<--This is important!
   _context.Exercises.Add(exercise);
}
await _context.SaveChangesAsync();