I have 3 tables, news,activities, threads.
threads contains all the replies for news and activities. Therefore
threads has a category_id and postid column to map a reply to a table
and initial thread.
How do i say that news has many threads, but threads has a generic
post_id foreign key field, not a news_id foreign key field as well as
a category_id of 1 (or 2 if its activities)?