Ufdjrw

I am learning MVC and for do this I am developing a "smart forum". I have do a database but I have some problem with entities. I have do this command

 "Scaffold-DbContext "Server=(localdb)mssqllocaldb;Database=SmartForum;Trusted_Connection=True; Microsoft.EntityFrameworkCore.SqlServer -OutputDir ModelsFromDb" ,

A code snippet:

modelBuilder.Entity<ArgomentiPerArea>(entity =>
 
 entity.HasKey(e => e.ArgomentoId);

 entity.Property(e => e.ArgomentoId).HasColumnName("argomentoId");

 entity.Property(e => e.Archiviato).HasColumnName("archiviato");

 entity.Property(e => e.AreaId).HasColumnName("areaId");

 entity.Property(e => e.ModeratoreId).HasColumnName("moderatoreId");

 entity.Property(e => e.NomeArgomento).HasColumnName("nome_argomento");

 entity.Property(e => e.NumeroRigaPerArea).HasColumnName("numero_riga_per_area");

 entity.Property(e => e.TestoPerArgomento).HasColumnName("testo_per_argomento");

 entity.HasOne(d => d.Area)
 .WithMany(p => p.ArgomentiPerArea)
 .HasForeignKey(d => d.AreaId)
 .HasConstraintName("FK_ArgomentiPerArea_Aree");

 entity.HasOne(d => d.Moderatore)
 .WithMany(p => p.ArgomentiPerArea)
 .HasForeignKey(d => d.ModeratoreId)
 .HasConstraintName("FK_ArgomentiPerArea_Moderatori");
 );

a second snippet :

public partial class ArgomentiPerArea
 
 public ArgomentiPerArea()
 
 Thread = new HashSet<Thread>();
 
 [Key]
 public int ArgomentoId get; set; 
 public string NomeArgomento get; set; 
 public int? AreaId get; set; 
 public bool? Archiviato get; set; 
 public int? NumeroRigaPerArea get; set; 
 public string TestoPerArgomento get; set; 
 public int? ModeratoreId get; set; 

 public virtual Aree Area get; set; 
 public virtual Moderatori Moderatore get; set; 
 public virtual ICollection<Thread> Thread get; set; 

 

public partial class Aree
 
 public Aree()
 
 ArgomentiPerArea = new HashSet<ArgomentiPerArea>();
 
 [Key]
 public int AreaId get; set; 
 public string NomeArea get; set; 
 public int? NumeroRiga get; set; 
 public int? NumeroColonna get; set; 

 public virtual ICollection<ArgomentiPerArea> ArgomentiPerArea get; set; 
 

public partial class Moderatori
 
 public Moderatori()
 
 ArgomentiPerArea = new HashSet<ArgomentiPerArea>();
 SegnalazioniPerModeratori = new HashSet<SegnalazioniPerModeratori>();
 
 [Key]
 public int ModeratoreId get; set; 
 public string UsernameModeratore get; set; 
 public string PasswordHash get; set; 
 public string NomeCognome get; set; 
 public bool? Archiviato get; set; 

 public virtual ICollection<ArgomentiPerArea> ArgomentiPerArea get; set; 
 public virtual ICollection<SegnalazioniPerModeratori> SegnalazioniPerModeratori get; set; 
 

when this code run

public class ArgomentiPerAreasController : Controller
 
 private ModelsFromDb.SmartForumContext db = new ModelsFromDb.SmartForumContext();

 // GET: ArgomentiPerAreas
 public ActionResult Index()
 
 var argomentiPerAreas = db.ArgomentiPerArea.Include(a => a.Area).Include(a => a.Moderatore); 

 string msg = "m";
 return View(argomentiPerAreas.ToList());
 

.............
.............

I check in view and "moderatore" and "area" have null value.
I don't understand but I know database first and MVC superficially. I hope in some suggestions.

This is likely due to circular references between your Argomenti* & the Moderator/Area. An Area holds a collection back to the Argomenti* so when MVC goes to serialize the root entity (Argomenti) it comes across the Area, then iterating through the area, a collection of Argomenti*, and co the cycle goes. It bails out and doesn't attempt to serialize the cyclical dependencies.

Generally the best thing to do with EF and views is not to attempt to send entities to the view. Instead, create a POCO (Plain old C# object) View Model to send to the view. This view model contains only the fields needed by the view, and your EF query uses .Select() to populate that view model. This avoids the whole cyclic reference issue, and negates the need for deliberate eager loading (.Include()) or the performance risk of lazy loading.

For example: If I want a list of Argumenti, and I want to display each Area and Moderator as part of that:

[Serializable]
public class ArgumentiViewModel

 public string NomeArgomento get; set; 
 public bool? Archiviato get; set; 
 public int? NumeroRigaPerArea get; set; 
 public string TestoPerArgomento get; set; 
 public string NomeArea get; set; // From Area
 public string NomeCognome get; set; // From Moderator

Then when I want to return this to the view:

var argomentiViewModels = db.ArgomentiPerArea
 .Select(x => new ArgomentiViewModel
 
 NomeArgomento = x.NomeArgomento,
 Archiviato - x.Archiviato,
 NumeroRigaPerArea = x.NumeroRigaPerArea,
 TestoPerArgomento = x.TestoPerArgomento,
 NomeArea = x.Area.NomeArea, // From Area
 NomeCognome = x.Moderatori.NomeCognome // From Moderator
 ).ToList();

string msg = "m";
return View(argomentiViewModels);

I sumarized a couple good reasons why code should not return Entities to the view here.