CSV containing 3000 UK postcodes, each with longitude and latitude. Also a rake task for populating your database.
rails g model Location postcode:string latitude:float longitude:float easting:string northing:string gridref:string town:string region:string
I've called my model 'Location' but it can be whatever you like. Note that if you change the name then you'll also need to edit the rake task. Remember to rake db:migrate once you've generated your model migration.
Copy the location.csv file into your app. I usually put mine in app/private_assets/location.csv. If you change the location then you'll also need to edit the rake task.
Copy the location.rake file into lib/tasks/location.rake. If you've changed the name of your model or the location of the csv file in steps 1 & 2 then you'll need to edit this rake file to reflect your changes.
rails rake location from the terminal is all you need to populate the database. If you're on Heroku then it's heroku run rake location --app MyAppName