rake task.rb

namespace :db do
	desc "Populate database"
	task :populate => :environment do	

 [Brand].each(&:delete_all)

	filename = File.join Rails.root, "databases/brands.csv"
	counter = 0
		CSV.foreach(filename, headers: true) do |row|
			p row
			brand = Brand.create!(id: row["id"], brandname: row["brandname"])
			counter += 1
		end 
		puts "Imported #{counter} brands"
  end
end 

model:

class Brand < ActiveRecord::Base
	validates :brandname, uniqueness: true
	# validates :brandname, uniqueness: { scope: :brandname } - in case you need it scoped
end

# also create a uniquenuess constraint in the database if needed

class MyMigration
	def change
		add_index :brands, :brandname, unique: true, null: false
	end
end