I am creating a web api that needs to return details about vehicles. The first part works fine, just returning data from my vehicles table. Now I have another table which may or may not contain additional data about vehicles in the first table. So when I get vehicle data, I want all of the vehicle data, and any additional data from the second table if it exists, like a left join in SQL.

Here are my classes (very much abridged for readability):

```
public class Vehicle
{
    [Key]
    [Required]
    public string registrationNo { get; set; }
    public string capacity{ get; set; }
    public string maxGross{ get; set; }
}

public class VehicleDvlaDetail
{
    [ForeignKey("Vehicle")]
    public string? registrationNumber { get; set; }
    public int? co2Emissions { get; set; }
}

```

And in my context class OnModelCreating I have (again, very abridged):

```
modelBuilder.Entity<Vehicle>(entity =>
        {
            entity.HasOne(dvlaRec => dvlaRec.dvlaDetail).WithMany().HasForeignKey(dvla => dvla.registrationNo);
        });

```

This works fine when there is an associated record in the DVLA table, but that isn't always the case. I need to keep them as separate entities as my API will be required to return details from the DVLA table separately as well. Is there any way to create an optional foreign key, as clearly, what I am doing is wrong.