Rails Migrations: tried to change the type of column from string to integer

Question

I created a table in my rails app with rails generate migrations command. Here is that migration file:

class CreateListings < ActiveRecord::Migration
  def change
    create_table :listings do |t|
      t.string :name
      t.string :telephone
      t.string :latitude
      t.string :longitude

      t.timestamps
    end
  end
end

Then I wanted to store the latitude and longitude as integers so I tried to run:

rails generate migration changeColumnType

and the contents of that file are:

class ChangeColumnType < ActiveRecord::Migration
  def up
    #change latitude columntype from string to integertype
    change_column :listings, :latitude, :integer
    change_column :listings, :longitude, :integer
    #change longitude columntype from string to integer type
  end

  def down  
  end
end

I was expecting the column type to change however the rake was aborted and the following error message appeared. I was wondering why this did not go through? Im using postgresql in my app.

rake db:migrate
==  ChangeColumnType: migrating ===============================================
-- change_column(:listings, :latitude, :integer)
rake aborted!
An error has occurred, this and all later migrations canceled:

PG::Error: ERROR:  column "latitude" cannot be cast to type integer
: ALTER TABLE "listings" ALTER COLUMN "latitude" TYPE integer

Tasks: TOP => db:migrate
(See full trace by running task with --trace)

NOTE: The table has no DATA. Thanks

Answer 1

I quote the manual about ALTER TABLE:

A USING clause must be provided if there is no implicit or assignment cast from old to new type.

What you need is:

ALTER TABLE listings ALTER longitude TYPE integer USING longitude::int;
ALTER TABLE listings ALTER latitude  TYPE integer USING latitude::int;

Or shorter and faster (for big tables) in one command:

ALTER TABLE listings
  ALTER longitude TYPE integer USING longitude::int
, ALTER latitude  TYPE integer USING latitude::int;

This works with or without data as long as all entries are valid as integer.
If the column has a DEFAULT, you may have to drop that (before the above) and recreate (after the above) for the new type.

Here is a blog article on how to do this with ActiveRecord.
Or go with @mu's advice in the comment. He knows his Ruby. I only know well about the PostgreSQL part.