We are not concerned about users. a one to one relation exist only between post and comment. i.e only one comment is allowed per post. i am just symbolising blog application with mine for easy understanding.
reg the UI, a list box with lot of fields from comments table (just for example) will limit users view to only one row but i want them to search the entire table both row wise and column wise before picking the required row, for establishing the link (between post and comment).