What to choose to use to determine the relationship?

I have 3 tables 1) table city (500 000 records) 2) table post (A city can have many posts) 3) table post_text (To store the text of the post)

table city has fields ID, NAME table post has fields ID, CITY_ID, POST_ID table post_text has fields TITLE, TEXT

What to use? Polymorphic? Through? Something else?

via Chebli Mohamed