EF Core returns null relationships before direct access

Question

I have several models like the ones below:

public class Mutant
{
    public long Id { get; set; }
    ...

    // Relations
    public long OriginalCodeId { get; set; }
    public virtual OriginalCode OriginalCode { get; set; }
    public int DifficultyLevelId { get; set; }
    public virtual DifficultyLevel DifficultyLevel { get; set; }
}

and

public class OriginalCode
{
    public long Id { get; set; }
    ...

    // Relations
    public virtual List<Mutant> Mutants { get; set; }
    public virtual List<OriginalCodeInputParameter> OriginalCodeInputParameters { get; set; }
}

and in OnModelCreating DBContexti made such a relationship:

        modelBuilder.Entity<Mutant>()
            .HasOne(m => m.OriginalCode)
            .WithMany(oc => oc.Mutants)
            .HasForeignKey(m => m.OriginalCodeId)
            .OnDelete(Microsoft.EntityFrameworkCore.Metadata.DeleteBehavior.Restrict);

        modelBuilder.Entity<Mutant>()
            .HasOne(m => m.DifficultyLevel)
            .WithMany(dl => dl.Mutants)
            .HasForeignKey(m => m.DifficultyLevelId)
            .OnDelete(Microsoft.EntityFrameworkCore.Metadata.DeleteBehavior.Restrict);

Now when I request Mutants, OriginalCode is null:

but as soon as I request OriginalCodeas shown below:

then the field of OriginalCodemutants will not be zero:

What is the reason and how can I fix this?

Answer 1

The reason is explained in the “ Download Related Data ” section of the EF Core documentation.

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var mutants = db.Mutants.Include(m => m.OriginalCode).ToList();

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Answer 2

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: ASP.net MVC Core 2 Entity Framework