How to load data from a different context in Entity Framework Core?

Question

I have two databases, the first for user names and their data, and the other database is for training, and I did not find a way to link the trainee from the training table and the users table in the first database for users. Two Contexts were used, but I did not find the solution.

CentralBase db context:

public partial class CentralBase : DbContext
{
    public CentralBase()
    {
    }

    public CentralBase(DbContextOptions<CentralBase> options)
        : base(options)
    {
    }

    public virtual DbSet<AspNetUser> AspNetUsers { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
#warning To protect potentially sensitive information in your connection string, you should move it out of source code. You can avoid scaffolding the connection string by using the Name= syntax to read it from configuration - see https://go.microsoft.com/fwlink/?linkid=2131148. For more guidance on storing connection strings, see https://go.microsoft.com/fwlink/?LinkId=723263.
        => optionsBuilder.UseSqlServer("data source =.\\SQLEXPRESS;initial catalog=CentralBase;integrated security=SSPI;TrustServerCertificate=True");

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<AspNetUser>(entity =>
        {
            entity.ToTable("AspNetUser");
            entity.HasKey(e => e.Id);
        });

        OnModelCreatingPartial(modelBuilder);
    }

    partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
}

Training db context:

public partial class Training : DbContext
{
    public Training()
    {
    }

    public Training(DbContextOptions<Training> options)
        : base(options)
    {
    }

    public virtual DbSet<TrainingPrograme> TrainingPrograme { get; set; }
    
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
#warning To protect potentially sensitive information in your connection string, you should move it out of source code. You can avoid scaffolding the connection string by using the Name= syntax to read it from configuration - see https://go.microsoft.com/fwlink/?linkid=2131148. For more guidance on storing connection strings, see https://go.microsoft.com/fwlink/?LinkId=723263.
        => optionsBuilder.UseSqlServer("data source =.\\SQLEXPRESS;initial catalog=Training;integrated security=SSPI;TrustServerCertificate=True");

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<TrainingPrograme>(entity =>
        {
            entity.ToTable("TrainingPrograme");
                entity.HasOne(p => p.AspNetUser)
                .WithMany(u => u.TrainingPrograme)
                .HasForeignKey(p => p.IdUserTraining);
        });


        OnModelCreatingPartial(modelBuilder);
    }

    partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
}

Table AspNetUser :

public partial class AspNetUser
{
    [Key]
    public string Id { get; set; } = null!;

    public string? UserName { get; set; }
    public virtual ICollection<EduQ2024.Areas.Train.Data.TableTraining.TrainingPrograme> TrainingPrograme { get; set; } = new List<EduQ2024.Areas.Train.Data.TableTraining.TrainingPrograme>();
}

Table TrainingPrograme:

public partial class TrainingPrograme
{
    public int Id { get; set; }

    public string? IdUserTraining { get; set; }

    public EduQ2024.data.TableCentral.AspNetUser AspNetUser { get; set; }
}

Test code

Data.DbTraining.Training Cn = new Data.DbTraining.Training();

var f = Cn.TrainingPrograme.ToList();
var g = f.Where(L => L.AspNetUser != null).ToList();.

Result:

AspNetUser = null