Cannot insert duplicate key. Composite Primary key. ASP.NET Entity Framework - asp.net

Question

I'm trying to create application using Entity Framework.
There's what I want to do (every entity has Id as well):
I want to use composite primary key in this case (PatientId + DiagnosisId).
There's a collection of Diagnoses in Patient model class:
public ICollection<Diagnosis> Diagnoses { get; set; }
public class Diagnosis
{
[Required]
[MaxLength(200)]
public String Type { get; set; }
public String Complications { get; set; }
public String Details { get; set; }
public Int32 DiagnosisId { get; set; }
public Patient Patient { get; set; }
public Int32 PatientId { get; set; }
}
Also in the database context I defined
public DbSet<Diagnosis> Diagnoses { get; set; }
and
modelBuilder.Entity<Diagnosis>().HasKey(x => new { x.DiagnosisId, x.PatientId });
in OnModelCreating method to create the composite primary key.
In an ASP.NET MVC CRUD controller I create Diagnosis and every time DiagnosisId is the same = 0. And I can't paste new data to database because it's like duplicate. That's my create post method on Pastebin if it could help

Answer 1

The parent key goes first:
modelBuilder.Entity<Diagnosis>().HasKey(x => new { x.PatientId, x.DiagnosisId });
Because you want all the Diagnoses for a particular Patent to be stored together.
And you need to request that the key is generated by the database. For EF Core its:
modelBuilder.Entity<Diagnosis>().Property(r => r.DiagnosisId).ValueGeneratedOnAdd();
for EF6 it's:
modelBuilder.Entity<Diagnosis>().Property(r => r.DiagnosisId).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);