EF Core 5 Duplicating Records in the Many-to-Many Relationship - visual-studio

Question

I'm having trouble implementing the many-to-many relationship using the Entity Framework Core 5 in Visual Studio.
I have the classes:
public class Medico
{
public Medico()
{
this.Especialidades = new HashSet<Especialidade>().ToList();
}
public int Id { get; set; }
public string Nome { get; set; }
public int CRM { get; set; }
public List<Especialidade>Especialidades { get; set; }
public class Especialidade
{
public int Id { get; set; }
public string Descricao { get; set; }
public IList<Medico>Medicos { get; set; }
}
And the Create method:
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("Id,Nome,CRM")] Medico medico)
{
var lstTags = Request.Form["chkTags"];
if (!string.IsNullOrEmpty(lstTags))
{
int[] splTags = lstTags.ToString().Split(',').Select(Int32.Parse).ToArray();
if (splTags.Count() > 0)
{
var medicoEspecialidades = await _context.Especialidades.Where(t => splTags.Contains(t.Id)).ToListAsync();
foreach (var me in medicoEspecialidades)
{
medico.Especialidades.Add(me);
}
}
}
if (ModelState.IsValid)
{
_context.Medicos.Add(medico);
await _context.SaveChangesAsync();
return RedirectToAction("Index");
}
return View(medico);
}
But when I run Create, it returns me with the following error:
"Cannot insert explicit value for identity column in table 'Especialidades' when IDENTITY_INSERT is set to OFF."
If I turn off the Identity_Insert of the Specialty table in the bank, it even inserts, but duplicates the records in the Specialty table.
I've been researching and trying to find a solution for 2 days now. Can someone who has been through this give me a hand?
The application source code is here: https://www.dropbox.com/s/xn6b95h7amfpuqa/AppCompleta%205.0.rar?dl=1

Answer 1

The approach looks Ok, though I would check to ensure that the medico being passed in does not have any Especialidade somehow coming in from the client as these would be detached entities. The error seems to imply that Medico may have a detached Especialidade in its collection. If the checked values represents everything that should be tracked, then this collection should be cleared and the Especialidade references added.
Do you have any explicit mapping configuration for either of these entities? If not I would highly recommend using one for Many-to-Many relationships as sometimes EF can default to unexpected schema assumptions when working off convention, especially in Code First if that is the case. I would look at your schema carefully to ensure it is matching what would be expected for a Many-To-Many. For example, what is the linking table name for Medico-Especialidade? Is there an entity defined for it in the configuration? This is entirely optional and EF should work it out, however if you do have explicit mapping that might not be configured correctly, tripping up the relationships.
One other detail giving off a smell:
public Medico()
{
this.Especialidades = new HashSet<Especialidade>().ToList();
}
public List<Especialidade>Especialidades { get; set; }
This should be:
public ICollection<Especialidade> { get; set; } = new HashSet<Especialidade>();
EF can work with lists, but when it comes to proxies and the behind the scenes EF is doing with entities it is generally better to declare your collection references as ICollection rather than concrete classes. ToListing a HashSet merely produces a List, so either = new HashSet<Especialidade>() or = new List<Especialidade>() will do. The difference would merely be the behaviour of the collection when you are populating it after "newing" up a Medico, or deserializing one.