Equivalent for .HasOptional in Entity Framework Core 1 (EF7)

You will not find an equivalent method in EF 7. By convention, a property whose CLR type can contain null will be configured as optional. So what decide if the relationship is optional or not is if the FK property is nullable or not respectively.

In summary, due to your Message_Id FK property is string, it already accepts null value, so if you use the following Fluent Api configuration:

modelBuilder.Entity<File>()
            .HasOne(s => s.Message)
            .WithMany()
            .HasForeignKey(e => e.Message_Id)

EF will configure your relationship as optional (or N : 0..1 as requested).

In case of your FK property is value type like int, you should declare it as nullable (int?).

Also I noticed now you have a navigation property with internal access modifier. You should always declare your entity properties as public.

In EF Core you can use two ways for relating two tables:

• Inside OnModelCreating:

  protected override void OnModelCreating(ModelBuilder modelBuilder)
  {
      base.OnModelCreating(modelBuilder);            
  
      modelBuilder.Entity<File>()
                  .HasOne(c => c.Message)
                  .WithOne()
                  .HasForeignKey(c => c.MessageId)                           
  }
  

• Create new class FileConfiguration and calling it inside OnModelCreating:

  public class FileConfiguration : IEntityTypeConfiguration<File>
  {
      public void Configure(EntityTypeBuilder<File> builder)
      {           
          builder.ToTable("File");            
  
          // Id
          builder.HasKey(c => c.Id);
          builder.Property(c => c.Id)
                 .ValueGeneratedOnAdd();
  
          // Message
          builder.HasOne(c => c.Message)
                 .WithOne(c => c.File)
                 .HasForeignKey<Message>(c => c.MessageId)
                 .OnDelete(DeleteBehavior.Restrict);
      }
  }
  

  and inside OnModelCreating put below codes:

  protected override void OnModelCreating(ModelBuilder modelBuilder)
  {
      base.OnModelCreating(modelBuilder);
  
      modelBuilder.ApplyConfiguration(new FileConfiguration());                                       
  }