Asp net - Delete record from db using OnDelete() - postgresql

Question

Trying to create simple CRUD app using Asp Net Core. I have 2 entities:
Department and Employee( one to many ). I need to delete record from Department table. But when Im trying to delete record using OnDelete(DeleteBehavior.Restrict) or OnDelete(DeleteBehavior.ClientSetNull) i have exception:
UPDATE or DELETE in table"Departments" violates foreign key constraint
"FK_Employees_Departments_DepartmentCode" table"Employees"
How can i fix this problem ?
Entity Employee:
public class Employee
{
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
[Required(ErrorMessage = "Input fullname of employee")]
public string FullName { get; set; }
[Required(ErrorMessage = "Input date of birth")]
public DateTime DateOfBirth { get; set; }
[Required(ErrorMessage = "Input code")]
public string Code { get; set; }
[Required(ErrorMessage = "Input fullname of employee")]
public int Salary { get; set; }
public string DepartmentCode { get; set; }
public Department Department { get; set; }
}
Entity Department:
public class Department
{
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
[Required(ErrorMessage = "Input name of department")]
public string Name { get; set; }
[Required(ErrorMessage = "Input code of department")]
public string Code { get; set; }
public ICollection<Employee> Employees { get; set; }
public Department()
{
Employees = new List<Employee>();
}
}
Context class settings:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Department>()
.HasMany<Employee>(d => d.Employees)
.WithOne(e => e.Department)
.HasForeignKey(e => e.DepartmentCode)
.OnDelete(DeleteBehavior.Restrict);
modelBuilder.Entity<Department>()
.HasKey(d => d.Code);
modelBuilder.Entity<Employee>()
.HasKey(e => e.Code);
modelBuilder.Entity<Department>()
.HasIndex(d => d.Name).IsUnique();
}

Answer 1

Of all fields in the DeleteBehavior enum only two actually add cascaded foreign key behavior to the database: Cascade and SetNull. All other options create foreign keys with no action on delete, but differ in what EF will do to its tracked entities.
In your case it should probably be SetNull because I assume that Employees can exist without Department. This setting will allow you to delete a Department object without loading its Employees. The database will set their DepartmentCode to null.

Answer 2

The delete behavior configured in EF can only be applied tho Entities that are tracked by EF change tracking. So you would need to load all Employees that belong to the department to make this work as expected.
BUT The database foreign key definition also defines the on delete action (cascading, set null, do nothing) So even if you code within your context a set null strategy, the DB still might apply different strategy for on delete. EF core defaults to cascade delete.