Adding an entity without knowing the parent id - entity-framework

Question

I need to insert a new entity without knowing it's PK. The parent entity has another property which is a guid and unique which is what we use to do cross db references and this is all I have. I have done it in the past but can't find a reference on how to do it again.
[Table("School")]
public class SchoolEntity
{
public SchoolEntity()
{
Students = new HashSet<StudentEntity>();
}
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
[Required]
public Guid ExternalId { get; set; }
[ForeignKey("SchoolId")]
public virtual ICollection<StudentEntity> Students { get; set; }
}
[Table("Student")]
public class StudentEntity
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
[Required]
public SchoolEntity School { get; set; }
}
//ExternalId won't work cause is not the primary key.
var school = new School { ExternalId = Guid.Parse('68e05258-550a-40f3-b68a-5d27a0d825a0') };
context.Attach(school);
context.Schools.Add.Add(new Student());
context.SaveChanges();

Answer 1

Well, the PK of the referenced entity is required in order to set properly the FK of the referencing entity.
If you don't have it, apparently you should find it (get it from the database) based on what you have (secondary identifier in your case). For instance:
var school = context.Schools.Single(e => e.ExternalId == externalId);
var student = new Student { School = school, ... };
context.Students.Add(student);
context.SaveChanges();
There is no way to get that working without fetching. If you don't want to fetch the whole referenced entity (and you are sure it's not tracked by the context), then you can fetch the PK only and Attach a stub entity:
var schoolId = context.Schools.Where(e => e.ExternalId == externalId)
.Select(e => e.Id).Single();
var school = new School( Id = schoolId);
context.Attach(school);
// ...