EF Code First Optional Navigation - entity-framework

Question

I have the following POCO objects:
public class Chat
{
public int ChatID { get; set; }
public virtual Audio ChatAudio { get; set; }
public virtual Video ChatVideo { get; set; }
}
public class Audio
{
public int AudioID { get; set; }
public int ChatID { get; set; }
public Chat Chat { get; set; }
public DateTime Recorded { get; set; }
}
public class Video
{
public int VideoID { get; set; }
public int ChatID { get; set; }
public Chat Chat { get; set; }
public DateTime Recorded { get; set; }
}
DBA has the following tables setup
-Chat-
ChatID
-Audio-
AudioID
ChatID <-- FK, Unique Index
Recorded
-Video-
VideoID
ChatID <-- FK, Unique Index
Recorded
So this is a 1-to-optional relationship. He doesn't want to create a nullable column for Audio or Video on the Chat table. How can I force this to work in EF Code First? The only way I've gotten this to work is to make those navigational properties collections and defined HasMany(). I do not want that. I want to be able to do:
if(Chat.Audio == null || Chat.Video==null)
{
// Do stuff
}
I appreciate any suggestions you may have.

Answer 1

No, you cannot map this model to your database schema with Entity Framework (no matter which version up to current version 4.2).
The main reason is that EF doesn't support unique key constraints, which means: mapping a reference navigation property to a column in the database which has a unique key constraint. EF will consider such a FK column in the database always as not unique and doesn't see the unique key constraint. So, the ends of the two associations in Chat must be collections. Or you could not expose them in your model, but internally EF will consider them always as "many"-ends.
The only true one-to-one relationship which EF supports are shared primary key relationships. It would mean that you don't have a ChatId column in the Audio and Video table but instead the primary key columns AudioId and VideoId are foreign key columns to the Chat table at the same time. Obviously the primary key columns cannot be all autogenerated identities in the database to make this kind of mapping possible.
A great description for both one-to-one mapping strategies and a comparison of their benefits and limitations can be found here:
Shared Primary Key Associations
One-to-One Foreign Key Associations

Answer 2

You could "cheat" and configure the the relationship as one to many, enforcing the one to one in the object model (also, in the datacontext initializer you can manually add the unique constraint to the database).
It would basically look something like this
public class Chat
{
public int ChatID { get; set; }
protected virtual ICollection<Audio> HiddenAudioCollection{get;set;}
public Audio ChatAudio
{
get{return HiddenAudioCollection.FirstOrDefault();}
set
{
if(HiddenAudioCollection==null)
HiddenAudioCollection=new List<Audio>();
if(HiddenAudioCollection.Any())
{
//decide if you throw an exception or delete the existing Audio
}
HiddenAudioCollection.Add(value);
}
}
//Do the same with ChatVideo
}

Answer 3

Another option, since the Video and Audio "belong" to the chat is to use a complex structure. Don't give the Video and Audio their own Id, they will be mapped to the chat table. I know that some DBA's hate this idea because it's optional data. But technically it's no different than storing an Address in a Customer table directly. It all depends on how normalized you want your database to be.

Answer 4

Let me make sure I understand this correctly:
Chat has zero or one Audio, and zero or one Video.
Audio, if it exists, belongs to exactly 1 Chat.
Video, if it exists, belongs to exactly 1 Chat.
Is this correct? If so, would your DBA mind a schema like this?
-Chat-
ChatID
-Audio-
ChatID <-- FK, Primary Key
Recorded
-Video-
ChatID <-- FK, Primary Key
Recorded
If so, I think you can do this:
public class Chat
{
public int ChatID { get; set; }
public virtual Audio ChatAudio { get; set; }
public virtual Video ChatVideo { get; set; }
}
public class Audio
{
public int ChatID { get; set; }
public virtual Chat Chat { get; set; }
public DateTime Recorded { get; set; }
}
public class Video
{
public int ChatID { get; set; }
public virtual Chat Chat { get; set; }
public DateTime Recorded { get; set; }
}
...
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Audio>().HasKey(a => a.ChatID);
modelBuilder.Entity<Video>().HasKey(a => a.ChatID);
var chat = modelBuilder.Entity<Chat>();
chat
.HasOptional(c => c.ChatAudio)
.WithRequired(a => a.Chat);
chat
.HasOptional(c => c.ChatVideo)
.WithRequired(a => a.Chat);
...
}