EF Core如何建立关系模型

Question

I'm a beginner on Asp.Net Core and I need to do de following:

I have the models curso and unidade , they share a many to many relationship.

Both models work fine by themselves, my problem however is that I can't make the relational model.

The idea is to make an endpoint that receives an object with one unidade and an array of curso and the API would add or remove the relationships accordingly.

The following code is what I have made so far, I'm getting an error Identity_Insert .

Am I on the right direction with this? Or is there another proper better way of doing this?

Modelo de curso
public class Curso
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    [Key]        
    public long curId { get; set; }

    [Required]
    [StringLength(80)]
    public string curDescricao { get; set; }

    [Required]
    [StringLength(1)]
    public string curStatus { get; set; }

    [StringLength(20)]
    public string curCodExterno { get; set; }

    [StringLength(60)]
    public string curObservacao { get; set; }
}

Modelo de unidade
public class Unidade
{        
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    [Key]
    public long uniId { get; set; }
    [Required]
    [StringLength(80)]        
    public string uniDescricao { get; set; }
    [Required]
    [StringLength(1)]
    public string uniStatus { get; set; }
    [StringLength(20)]
    public string uniCodExterno { get; set; }
    public byte[] uniImagem { get; set; }
}

Modelo de CursoUnidade
public class CursoUnidade
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    [Key]
    public long cuuId { get; set; }

    /*[Required]        
    public long cuuCurId { get; set; }
    [ForeignKey("cuuCurId")]*/        
    public List<Curso> Curso { get; set; }

    /*[Required]
    public long cuuUniId { get; set; }
    [ForeignKey("cuuUniId")]        */
    public Unidade Unidade { get; set; }
}

Serviço de unidade
public void AddTeste(CursoUnidade cursoUnidade)
{
    _contexto.Add(cursoUnidade);
    _contexto.SaveChanges();
}

Answer 1

The problem is that the Curso and Unidade instances on the CursoUnidade that's being passed in are not being tracked by EF. As a result, when you attempt to add it and save, it's attempting to create those two entities again. However, since they both already have ids, it cannot do that and fails.

This is one of the many problems associated with trying to save what's passed into your action directly. You should use a view model and then map that over the the actual entity you end up saving. This forces you to be explicit about what you're doing and results in less bugs such as this one. Also, unless you're actually intending the two related instances to be able to modified at the same time they're being associated, it doesn't make sense to post the entire entity, anyways. This can easily lead to an overpost exploit, where the user makes modifications to entities they shouldn't be able to make, and because of the sloppy handling of the post server-side, those just get blindly saved to the database.

Instead, use a view model/DTO like:

public class CursoUnidadeDTO
{
    public long CursoId { get; set; }
    public long UnidadeId { get; set; }
}

Then, in your action:

public void AddTeste(CursoUnidadeDTO cursoUnidade)
{
    var curso = await _contexto.Cursos.FindAsync(cursoUnidade.CursoId);
    var unidade = await _contexto.Unidades.FindAsync(cursoUnidade.UnidadeId);
    if (curso == null || unidade == null)
    {
        return BadRequest("Cannot create relationship");
    }

    _contexto.Add(new CursoUnidade { Curso = curso, Unidade = unidade });
    await _contexto.SaveChangesAsync();
}

Or, if your CursoUnidade class had explicit foreign key properties, you wouldn't even need to look anything up:

_contexto.Add(new CursoUnidade { CursoId = cursoUnidade.CursoId, UnidadeId = cursoUnidade.UnidadeId });

Alternatively, you can simply attach these entities first, so that EF knows about them:

_contexto.Attach(cursoUnidade.Curso);
_contexto.Attach(cursoUnidade.Unidade);
_contexto.Add(cursoUnidade);
await _contexto.SaveChangesAsync();

That will solve your immediate issue, but like I said previously, it will also allow overposts, and if that's not something you're explicitly allowing, could be very dangerous.