Question

rake db:seed

Does rerunning seed data drop the existing seed data and re-create or adds just the new records whenever you modify the seed data?

Answer 1

When you run rake db:seed, db/seeds.rb is only included with the rails environment.
Therefore, nothing else is done on your database than what you mention it to do.

I usually put the following at the top of my db/seeds file :

Dir['app/models/**/*.rb'].each do |model|
    model_object = model.camelize.constantize
    model_object.delete_all
end

This way, all previous seed data is removed, and I get this fresh new proper data I want in my dev environment.