Use
UPDATE: move logic code to model
# company.rb
def company_with_country
"#{name} - #{country.name}" # this is better than using `string + string` since it creates only 1 string
end
# view
collection_select :department, :company_id, Company.includes(:country).all, :id, :company_with_country
UPDATE: faster version because it only uses needed columns
# controller
@companies = Company.joins(:country)
.select('companies.id, companies.name, countries.name AS country_name')
.map { |c| ["#{c.name} - #{c.country_name}", c.id] }`
# view
select :department, :company_id, @companies