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 is integer. I have read about migration and it has available transformations:

• rename_column(table_name, column_name, new_column_name): Renames a column but keeps the type and content.
• change_column(table_name, column_name, type, options): Changes the column to a different type using the same parameters as add_column.

So, I create my migration file like this:

class RenameSemesterFromGeneralExams < ActiveRecord::Migration    def change     rename_column :general_exams, :semester, :semester_id     change_column :general_exams, :semester_id, :integer   end end 

But, when I run rake db:migrate, it has error:

==  RenameSemesterFromGeneralExams: migrating ================================= -- rename_column(:general_exams, :semester, :semester_id)    -> 0.0572s -- change_column(:general_exams, :semester_id, :integer) rake aborted! An error has occurred, this and all later migrations canceled:  PG::Error: ERROR:  column "semester_id" cannot be cast to type integer : ALTER TABLE "general_exams" ALTER COLUMN "semester_id" TYPE integer 

In my table GeneralExam, I destroyed all data. So, anyone can tell me how can I do that? Or I must create two migration files?

Answer 1

This works as of Rails 4

def change   rename_column :general_exams, :semester, :semester_id   change_column :general_exams, :semester_id, :integer end 

Answer 2

Your problem is probably that the semester contains data that cannot be converted to integers. That's why you get a cast error.

I suspect you need to do more work to make this work as the only thing that comes to mind is removing the column and creating a new one with the correct values.

But you can simply remove_column and then add_column in one migration. That should work flawlessly.

I'd also suggest you only add_column first, then do the mapping process where you map the old semester value onto the new semester_id and then drop the column.

Keep in mind that you can do ActiveRecord manipulations inside your migration. So you can put that code in there.