Constants in composite foreign keys

[hauntingEcho]

What problem are you trying to solve?

I've recently inherited a codebase which makes heavy use of the pattern:

class Address {
  int AddressID;
  int RefType;
  int RefID;
  string Address;
}
class Customer {
  int CustomerID;
}
class Contact {
  int ContactID;
}
enum RefType {
  Customer = 1,
  Contact = 2,
  User = 3,
  // ...
}

I'd like to add a link from the Customer to their Address, meaning that the expected pattern would be to either:

• use a discriminator. They're already using context.Addresses pretty heavily across multiple repos, so this would break everywhere that they're manually setting RefType as well as breaking for undocumented values of RefType that crop up. The changes to discriminator behavior needed to make this work seem like they'd be a much bigger request to make.
• use something like the following:

entity
  .HasOne(c => c.Address)
  .HasOne()
  .HasForeignKey<Address>(a => new { a.RefID, a.RefType })
  .HasPrincipalKey<Customer>(c => new { c.CustomerID, RT = (int)RefType.Customer });

related StackOverflow

Describe the solution you'd like

The ability to use a constant value for one or more properties in the composite forms of HasForeignKey and HasPrincipalKey

[AndriySvyryd]

This will be possible when #7623 is implemented.

[angelaki]

@ajcvickers, @AndriySvyryd or anyone else, just to make sure, is this still correct for EFCore 9.0? I'm having a quite simple use-case I tought:

I'm having a table to store key/value pairs for any objects (string Module, Guid Id, string Name, string Value). Now I tought I'd be able to make the relations this way:

a.HasMany(p => p.Tags).WithOne().HasPrincipalKey(p => new { Module = Module.Name, p.Id }).HasForeignKey(p => new { p.Module, p.Id }); but EFCore brings an System.ArgumentException because of the constant value.

Ok, I could say Guids won't overlap anyway but I'd rather have a clean join here.