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.

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.

Answer 3

I hope this help

class ModifyColumnTables 
  def change
    remove_column :posts, :old_column
    add_column :posts, :new_column, :type_of_column
  end
end

Answer 4

This works as of Rails 4

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