Reset the database (purge all), then seed a database
I use rake db:reset which drops and then recreates the database and includes your seeds.rb file.http://guides.rubyonrails.org/migrations.html#resetting-the-database
You can delete everything and recreate database + seeds with both:
rake db:reset: loads from schema.rbrake db:drop db:create db:migrate db:seed: loads from migrations
Make sure you have no connections to db (rails server, sql client..) or the db won't drop.
schema.rb is a snapshot of the current state of your database generated by:
rake db:schema:dump
As of Rails 5, the rake commandline tool has been aliased as rails so now
rails db:reset instead of rake db:reset
will work just as well