Allow database default values for FK properties that are part of a composite key

[ajcvickers]

Fixes #27299

[AndriySvyryd]

Wouldn't it make sense to also stop warning on composite PKs that don't contain an FK?

[localhost09]

Coming back to this issue #27299 which was fixed with this merge.
It appears to work for the top level entities, but not for child entities.
Adding to the context works, saving results in exception.

Setup:

• BaseModel which all models are inheriting from. Is used primarily for row isolation between tenants.
• Besides the FK, child entity B has a PK (composite Key to ensure uniqueness within a tenant)
• The TenantId is injected using a Valuegenerator (which is called when adding to context as expected)

BaseModel:

public abstract class BaseModel
{
        public virtual Tenant Tenant { get; set; }
        public virtual string TenantId { get; set; }
}

//Inherited Entity A

public class A : BaseModel
{
        [Key]
        [MaxLength(36)]
        public string SomeKeyField {get; set; }
        public string Name { get; set; }
        public virtual ICollection<B> ChildListOfB { get; set; }
}

//Child Entity B

public class B : BaseModel
{
        public string SomeExtId { get; set; }
        public string SomeOtherValue { get; set; }
}

//Tenant Entity 
 public class Tenant 
{
        [Key]
        [MaxLength(36)]
        public string TenantId {get; set; }
     
}

// OnModelCreating showing for B for now - same for all entities 

modelBuilder.Entity<B>().HasKey(pi => new { pi.TenantId, pi.SomeExtId, pi.SomeOtherValue });
modelBuilder.Entity<B>().Property(e => e.TenantId).HasValueGenerator<VG>();

//Value Generator (VG) 

    public class VG: ValueGenerator<string>
    {

       public override bool GeneratesTemporaryValues => false;

        public override string? Next(EntityEntry entry)
        {

            var s = (entry.Context as OwnDbContext);
            if (s != null)
                return s.SomeProp.GetTenantId();
            return null;
        }
    }

Exception:

The value of 'B.TenantId' is unknown when attempting to save changes. This is because the property is also part of a foreign key for which the principal entity in the relationship is not known.'

Current workaround:
Overriding SaveChangesAsync and adding the value explicitly

entityEntry.CurrentValues["TenantId"] = _internalField.GetTenantId();

Versions
Microsoft.EntityFrameworkCore 7.0.0
Target Framework: net6

[ajcvickers]

@localhost09 Please open a new issue and attach a small, runnable project or post a small, runnable code listing that reproduces what you are seeing so that we can investigate.