$employee = employee::find(1);
$salary1 = new Salary;
$salary->amount = '123456789';
$salary->payment_date = '15/07/2020';
$salary2 = new Salary;
$salary->amount = '123456789';
$salary->payment_date = '16/07/2020';
$employee = $employee->salary()->saveMany([$salary1, $salary2]);