Why the Left Outer join?

Question

weird one. (Probably not weird, at all)

I have 3 objects, Employee, Rota and Department.

public class Employee
{
    public int Id { get; set; }
    publ         


        

Answer 1

The reason is incorrect mapping. It looks correct but it is not. Use these instead:

internal class EmployeeMapping : EntityTypeConfiguration
{
    public EmployeeMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("UserId");

        HasRequired(a => a.Department).WithMany()
                                                  .Map(a => a.MapKey("DepartmentId"));
    }
}

internal class RotaMapping : EntityTypeConfiguration
{
    public RotaMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("RotaId");

        HasOptional(a => a.Employee).WithMany()
                                              .Map(a => a.MapKey("EmployeeId"));
        HasOptional(a => a.Department).WithMany()
                                                  .Map(a => a.MapKey("DepartmentId"));
    }
}

Your mapping is correctly interpreted when creating database and database looks correct but EF thinks that you map all relations as one-to-one. That confuse EF and it will generate queries used for one-to-one to create internal entity references. These left joins are necessary for one-to-one relation when you tell EF that dependent entities are optional - EF doesn't know if they exist unless it loads their keys.