How to rename a column and change its type by migration same time

Question

In my general_exams table, I have a column named semester, type is string. Now I want to change its name to semester_id, type

Answer 1

This error is because there is existing data in the tables (or default values, perhaps ..) that PG doesn't know how to convert from string to integer. Either get rid of the data or tell PG how you want to convert it, using PG specific SQL (I think you'll want USING) and execute migration command. See Rails guides on migrations.