search for: how_old

Hi all, I have a has_many relation between Parent and Child Parent p Child c p.children << c => create the relationship in the database (why not need p.save to see the child add?) why when i modifiy a field in c.how_old i have to explicity call to c.save if i want to see the modification in database (p.save doesn''t persist the modification of chlild c) because when i modify the child c and apply p.save it does nothing !!! Other question Why p.children << c doesn''t need to do p.save to see...