Use the proper annotations for objects in .NET Core's Code First Entity Framework stuff.

Consider these objects:

1. using System;
   using System.ComponentModel.DataAnnotations;
   using System.ComponentModel.DataAnnotations.Schema;
   namespace FluffyNothing.Core.Objects
   {
      public class Game
      {
         [Key]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
         public long GameId { get; set; }
         public long Player1 { get; set; }
         [Required]
         public string Player1Hand { get; set; }
         public DateTime Player1Time { get; set; }
         public long? Player2 { get; set; }
         public string Player2Hand { get; set; }
         public DateTime? Player2Time { get; set; }
      }
   }
    
2. using System;
   using System.ComponentModel.DataAnnotations;
   using System.ComponentModel.DataAnnotations.Schema;
   namespace FluffyNothing.Core.Objects
   {
      public class Message
      {
         [Key]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
         public long MessageId { get; set; }
         [Required]
         [ForeignKey("PlayerId")]
         public Player Player { get; set; }
         [Required]
         public string Copy { get; set; }
         public DateTime Time { get; set; }
      }
   }
    
3. using System.Collections.Generic;
   using System.ComponentModel.DataAnnotations;
   using System.ComponentModel.DataAnnotations.Schema;
   namespace FluffyNothing.Core.Objects
   {
      public class Player
      {
         [Key]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
         public long PlayerId { get; set; }
         [Required]
         [MaxLength(15)]
         public string PlayerIp { get; set; }
         public virtual ICollection<Message> Messages { get; set; }
      }
   }
    

It makes this at the database:

Things to note here include how to make a parent/child relationship, how to make a varchar be a specific width, and how making a varchar not nullable takes a different shape than that of other types. In the case of long and DateTime we just either have a nullable type at the C# side or we don't, right? In the case of string we have to use the Required attribute. I guess this much will change in C# 8 with the introduction of nullable strings.