How do I model a 'has a' database relationship with multiple principles? - database

Question

In code I have an object graph that looks like this:
public class Author
{
public int AuthorId { get; set; }
public string Name { get; set; }
public Address Address { get; set; }
}
public class Publisher
{
public int PublisherId { get; set; }
public string Name { get; set; }
public Address Address { get; set; }
}
public class Address
{
public int AddressId { get; set; }
public string Postcode { get; set; }
}
How do I model this in a relational database schema?
This needs to be a 0-1..1 relation, i.e. Authors and Publishers may either have or not have an Address.
Ideally Addresses would have to be referenced by either an Author or a Publisher but not both.
Massive bonus respect if you can model it in Entity Framework Core using navigation properties and have cascade delete remove the Address when an Author or Publisher is removed. (But I'm betting no one will be able to).
I'd list all the things I've tried, but the post would be so long no one would read it. It'll be quicker just to say I've tried everything I can think of.

Answer 1

There are many ways you can achieve the goal in EF Core. The key point is that Address will be the dependent end of the relationships and it will contain optional FKs to the principal entities Author and Publisher.
Here are the possible Address models and configurations:
(1) Address with explicit FK and navigation properties
Model:
public class Address
{
public int AddressId { get; set; }
public string Postcode { get; set; }
public int? AuthorId { get; set; }
public Author Author { get; set; }
public int? PublisherId { get; set; }
public Publisher Publisher { get; set; }
}
Configuration:
modelBuilder.Entity<Author>()
.HasOne(e => e.Address)
.WithOne(e => e.Author)
.HasForeignKey<Address>(e => e.AuthorId)
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<Publisher>()
.HasOne(e => e.Address)
.WithOne(e => e.Publisher)
.HasForeignKey<Address>(e => e.PublisherId)
.OnDelete(DeleteBehavior.Cascade);
(2) Address with navigation properties only
Model:
public class Address
{
public int AddressId { get; set; }
public string Postcode { get; set; }
public Author Author { get; set; }
public Publisher Publisher { get; set; }
}
Configuration:
modelBuilder.Entity<Author>()
.HasOne(e => e.Address)
.WithOne(e => e.Author)
.HasForeignKey<Address>("AuthorId")
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<Publisher>()
.HasOne(e => e.Address)
.WithOne(e => e.Publisher)
.HasForeignKey<Address>("PublisherId")
.OnDelete(DeleteBehavior.Cascade);
(3) Address with explicit FK properties only
Model:
public class Address
{
public int AddressId { get; set; }
public string Postcode { get; set; }
public int? AuthorId { get; set; }
public int? PublisherId { get; set; }
}
Configuration:
modelBuilder.Entity<Author>()
.HasOne(e => e.Address)
.WithOne()
.HasForeignKey<Address>(e => e.AuthorId)
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<Publisher>()
.HasOne(e => e.Address)
.WithOne()
.HasForeignKey<Address>(e => e.PublisherId)
.OnDelete(DeleteBehavior.Cascade);
(4) Address without explicit FK and navigation properties
Model:
public class Address
{
public int AddressId { get; set; }
public string Postcode { get; set; }
}
Configuration:
modelBuilder.Entity<Author>()
.HasOne(e => e.Address)
.WithOne()
.HasForeignKey<Address>("AuthorId")
.OnDelete(DeleteBehavior.Cascade);
modelBuilder.Entity<Publisher>()
.HasOne(e => e.Address)
.WithOne()
.HasForeignKey<Address>("PublisherId")
.OnDelete(DeleteBehavior.Cascade);
Reference: Relationships