EF Core insert explicit value for identity column

Question

I have problem with inserting data to database table which has foreign keys. As error I'm getting

Cannot insert explicit value for identity column in table 'MachineTypes' when IDENTITY_INSERT is set to OFF.Cannot insert explicit value for identity column in table 'SpareTypes' when IDENTITY_INSERT is set to OFF.

BaseEntity.cs

public abstract class BaseEntity
{
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public Int64 Id { get; set; }

    public DateTime CreateDate { get; set; }
}

MachineType.cs

public class MachineType : BaseEntity
{
    [Required]
    [StringLength(50)]
    public string Name { get; set; }
}

SpareType.cs

   public class SpareType : BaseEntity
    {
        [Required]
        [StringLength(25)]
        public string Name { get; set; }
    }

SparePart.cs

public class SparePart : BaseEntity
{
[Required]
[StringLength(100)]
public string InternalCode { get; set; }

[StringLength(4096)]
public string Description { get; set; }

[StringLength(255)]
public string NameOnFolder { get; set; }

public decimal? Enter { get; set; }

public decimal? Exit { get; set; }

public decimal? Thickness { get; set; }

public string Band { get; set; }

public string Color { get; set; }

public bool Elastic { get; set; }

[Required]
public virtual MachineType MachineType { get; set; }

[Required]
public virtual SpareType SpareType { get; set; }
}

SparePartViewModel.cs

 public class SparePartViewModel
    {
        public int Id { get; set; }

        public DateTime CreateDate { get; set; }

        [Required]
        [StringLength(100)]
        public string InternalCode { get; set; }

        [StringLength(4096)]
        public string Description { get; set; }

        [StringLength(255)]
        public string NameOnFolder { get; set; }

        public decimal? Enter { get; set; }

        public decimal? Exit { get; set; }

        public decimal? Thickness { get; set; }

        public string Band { get; set; }

        public string Color { get; set; }

        public bool Elastic { get; set; }



        [Required]
        public virtual MachineType MachineType { get; set; }

        [Required]
        public virtual SpareType SpareType { get; set; }
    }

Controller for posting

[Route("/api/v1/items")]
public class SparePartController : Controller
{
private IRepository<SparePart> _repoSparePart;


public SparePartController(IRepository<SparePart> repoSparePart)
{
    _repoSparePart = repoSparePart;

}

[HttpPost("")]
public async Task<IActionResult> Post([FromBody]SparePartViewModel viewModel)
{
    if (ModelState.IsValid)
    {
        var newItem = Mapper.Map<SparePart>(viewModel);
        newItem.CreateDate = DateTime.Now;

        _repoSparePart.Insert(newItem);

        if (await _repoSparePart.SaveChangesAsync())
        {
            return Created($"items/{newItem.InternalCode}", Mapper.Map<SparePartViewModel>(viewModel));
        }
    }
    return BadRequest("Failed to save.");
}

}

AppContext.cs

   public class AppContext : IdentityDbContext<ApplicationUser>
    {
        private IConfigurationRoot _config;

        public AppContext(IConfigurationRoot config, DbContextOptions options) : base(options)
        {
            _config = config;
        }

        public DbSet<SpareType> SpareTypes { get; set; }
        public DbSet<MachineType> MachineTypes { get; set; }
        public DbSet<SparePart> SpareParts { get; set; }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            builder.Entity<SpareType>()
                .HasIndex(s => new { s.Name })
                .IsUnique(true);

            builder.Entity<MachineType>()
                .HasIndex(s => s.Name)
                .IsUnique(true);


            base.OnModelCreating(builder);
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            base.OnConfiguring(optionsBuilder);
            optionsBuilder.UseSqlServer(_config["ConnectionStrings:RndDbContextConnection"]);
        }
    }

During the posting all the data are catch correctly, but when should be inserted into database I'm getting an error.

Answer 1

When inserting the new SparePart instance, the MachineType and SpareType references need to be fetched from the same DbContext beforehand. Otherwise EF thinks you are trying to create a new MachineType and SpareType. And because the Id field is set, the database won't allow the insert. Something like so:

newItem.MachineType = _context.MachineTypes.Find(<some_id>);
newItem.SpareType = _context.SpareTypes.Find(<some_id>);
context.SpareParts.Add(newItem);
context.SaveChanges();

Another thing you can do is expose the foreign key on your model, then it is enough to set it on the new instance before adding it to the DbSet. In SparePart:

[ForeignKey("MachineType")]
public Int64 MachineTypeId { get; set; }

[ForeignKey("SpareType")]
public Int64 SpareTypeId{ get; set; }