How do I change a string column into a bigint?

Postgres is telling you that there is existing data in that column which it doesn't know how to convert, so it needs an ALTER statement which supplies a USING clause for the column to specify how to cast existing values.

Unfortunately, you're going to need to drop down the database-specific code to accomplish this, or use something similar to the solution suggested here:

http://webjazz.blogspot.co.uk/2010/03/how-to-alter-columns-in-postgresql.html

Edit: Here's how you might do it directly in SQL in your migration:

execute <<-SQL
  ALTER TABLE ip_to_countries
  ALTER COLUMN ip_number_from TYPE bigint USING ip_number_from::bigint
SQL

What is in your ip_number_from column?

In any case, I would probably:

• create a new column of type bigint,
• copy the data from ip_number_from to new_column manually through rails console or a rake task,
• drop the original ip_number_from column
• rename new_column to ip_number_from

Or you could drop down to SQL, like mjtko suggested, but I'm not sure it will be any easier.

Update

I looked up what Yule is suggesting. I think it would be a bit dangerous to do all of this in one migration, since you can't really check if the casting/copying of data was successful. If you did want to do it in one migration, in your case it would look something like this:

def up
  add_column :table, :new_column, :bigint
  Model.reset_column_information
  Model.all.each do |m|
    m.update_attribute :new_column, Model.bigint_from_ip_number_from(m)
  end
  drop_column :table, :ip_number_from
  rename_column :table, :new_column, :ip_number_from
end

You should also then add a corresponding down migration.

You can always split this up in multiple migrations and check the progress/success as you go.