bulk_updating.cs

// My first attempt
var appointments = _context.T_APPOINTMENT.Where(c => ids.Contains(c.CUSTOMER_ID));

foreach (var appointment in appointments)
{
	appointment.CUSTOMER_ID = newCustomer.CUSTOMER_ID;
	_context.T_APPOINTMENT.ApplyCurrentValues(appointment);
}
_context.SaveChanges();

// My second, optimized for performance, attempt
var query = string.Format("Update T_APPOINTMENT SET CUSTOMER_ID = {0} where CUSTOMER_ID in ({1})",
                          newCustomer.CUSTOMER_ID,
                          string.Join(",", ids));

_context.ExecuteStoreCommand(query);
_context.SaveChanges();