EF Core Complex Object Insertion with Webapi

Question

I am developing a Webapi (.NET) application with Entityframework Core. I implemented the following entities:

Pais.Cs

public class Pais
{
    [Key]
    public int Id { get; set; }
    [MaxLength(100), Required]
    public string Descricao { get; set; }
}

Estado.Cs

public class Estado
{
    [Key]
    public int Id { get; set; }
    [Required]
    public virtual Pais Pais { get; set; }
    [Required]
    public string Descricao { get; set; }
    public string Sigla { get; set; }
}

City.Cs

public class Cidade
{
    [Key]
    public int Id { get; set; }
    public virtual Estado Estado { get; set; }
    public string Descricao { get; set; }
}

When trying to enter a new record with JSON below:

{
    "descricao": "Ribeirão Preto",
    "estado": {
        "id": 1,
        "pais": {
            "id": 1,
            "descricao": "Brasil"
        },
        "descricao": "São Paulo",
        "sigla": "SP"
    }
}

I have the following return Cannot insert explicit value for identity column in table 'Pais' when IDENTITY_INSERT is set to OFF.

It seems to me the system is trying to insert the Country that is related to the city of JSON, but I’m not sure why.

Answer 1

Entityframework is trying to add a new Pais, and as it is not explicit that it is the EF that should create the Ids, the EF is waiting for the Country Id to be null or 0, thus passing the responsibility of the repository to create this Id.

However, this is not your belief. You just want to add a Cidade. A workaround is, before adding the new record Cidade in the RU, make a query for Estado and binds to the new object:

var estado = dbContext.Estados.Single(estado => estado.Id == novaCidade.Estado.Id);
novaCidade.Estado = estado;
dbContext.Cidades.Add(novaCidade);
dbContext.SaveChanges();

So your EF will know that the new Cidade has direct relation to an existing parent record in your repository.

But to really settle all this, review how you are setting up your EF. Consider maybe, or add a EstadoId in class Cidade, or create different repository models within the scope of the repository - which will require great effort in development, but the result is great.