Based on following models
class Company < ActiveRecord::Base
belongs_to :country
end
class Country < ActiveRecord::Base
has_many :companies
end
collection_select
should do the trick for you:
collection_select(:company, :country_id, Country.all, :id, :name, :prompt => 'Please select country')
The above code assumes that the countries table have a name
column. If it doesn't, replace the fifth parameter with whatever the column of the country name is.
:prompt
means that you're forcing the user to choose one country.
Find more information in the Rails API documentation.