I am using .NET 6 to develop EF Core MVC program, The concept is single User can have multiple roles, and the role data is create by a User from another system.

I have a base class EntityBase.cs :

public abstract class EntityBase
{
    public int Id { get; set; }
}

And a base class CreatableEntity.cs inherited EntityBase.cs again

public abstract class CreatableEntity : EntityBase
{
    public int CreatedById { get; set; }
    public virtual User CreatedBy { get; set; }
}

Then have a User.cs class inherited CreatableEntity class

public class User : CreatableEntity
{
    public string Username { get; set; }

    public string Password { get; set; }

    public virtual ICollection<Role> Roles { get; set; }

    public virtual ICollection<UserRole> UserRoles { get; set; }
}

And have a Role.cs class inherited CreatableEntity class also

public class Role : CreatableEntity
{
    public string RoleName { get; set; }

    public virtual ICollection<User> Users { get; set; }

    public virtual ICollection<UserRole> UserRoles { get; set; }
}

And UserRole.cs class is the table to store "UserId" and "RoleId"

public class UserRole
{
    public int UserId { get; set; }
    public virtual User Users { get; set; }

    public int RoleId { get; set; }
    public virtual Role Roles { get; set; }
}

The OnModelCreating in my context class

public class TestContext : DbContext
{     
    public DbSet<Role> Roles { get; set; }
    public DbSet<User> Users { get; set; }
    public DbSet<UserRole> UserRoles { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Role>()
            .HasBaseType((Type)null)
            .HasKey(rb => rb.CreatedById);

        modelBuilder.Entity<User>()
           .HasMany(u => u.Roles)
           .WithMany(u => u.Users)
           .UsingEntity<UserRole>(
               j => j
                   .HasOne(ua => ua.Roles)
                   .WithMany(a => a.UserRoles)
                   .HasForeignKey(ua => ua.UserId),
               j => j
                   .HasOne(ua => ua.Users)
                   .WithMany(u => u.UserRoles)
                   .HasForeignKey(ua => ua.RoleId)
               j =>
               {
                   j.HasKey(ua => new { ua.RoleId, ua.UserId });
               });
    }
}

When I run the program, hit the following error:

Unable to determine the relationship represented by navigation 'Role.CreatedBy' of type 'User'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

I researching some time and found a article from Microsoft :