Entity Framework Core - 3 tier relationship - entity-framework-core

Question

I have to apply a set of relationships with a system that incorporates a messaging system.
I have the two of my domain object with one mapping object (for the many-to-many relationship):
public class User
{
public User()
{
UserMails = new List<UserMail>();
}
public int Id { get; set; }
public ICollection<UserMail> UserMails { get; set; }
}
public class Mail
{
public Mail()
{
UserMails = new List<UserMail>();
}
public int Id { get; set; }
public string Title { get; set; }
public string Body { get; set; }
public ICollection<UserMail> UserMails { get; set; }
}
public class UserMail
{
public int Id { get; set; }
public int FromUserId { get; set; }
public User FromUser { get; set; }
public int ToUserId { get; set; }
public User ToUser { get; set; }
public int MailId { get; set; }
public Mail Mail { get; set; }
}
How would I configure this relationship using Fluent API such that there's a many to many relationship between User and Mail and Mail can have 2 foreign keys back to the UserFrom and UserTo?
Any help on this would be greatly appreciated.

Answer 1

If you are trying to model the relationship between a mail and its sender/recipient, then you don't need a many-to-many relation, or 2 foreign keys in your joining entity. Instead, you need 2 one-to-many relations like below -
public class User
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<Mail> ReceivedMails { get; set; }
public ICollection<Mail> SentMails { get; set; }
}
public class Mail
{
public int Id { get; set; }
public string Title { get; set; }
public string Body { get; set; }
public int SenderId { get; set; }
public User Sender { get; set; }
public int RecipientId { get; set; }
public User Recipient { get; set; }
}
and you can configure them as -
protected override void OnModelCreating(ModelBuilder builder)
{
builder.Entity<Mail>()
.HasOne(p => p.Sender)
.WithMany(p => p.SentMails)
.HasForeignKey(p => p.SenderId)
.OnDelete(DeleteBehavior.NoAction);
builder.Entity<Mail>()
.HasOne(p => p.Recipient)
.WithMany(p => p.ReceivedMails)
.HasForeignKey(p => p.RecipientId)
.OnDelete(DeleteBehavior.NoAction);
}