Lets suppose that a file named product.csv exists into db folder in our applications. To import this file we will create file import.rake in my_app/lib/tasks/.

The import.rake file looks like...

require 'csv'
desc "Import product from csv file"
  task :import => [:environment] do
   file = "db/product.csv"
    CSV.foreach(file, headers: true) do |row|
      product_hash = row.to_hash
      product = Product.where(id: product_hash["id"])

      if product.count == 1
        product.first.update_attributes(product_hash)
      else
        Product.create!(product_hash)
      end
  end 
end

The above code reads the row from browsed csv file and inserting it to table products if record not in table, if the record exist then it'll update.

The contents of a csv file which one to be import

Path:  my_app/product.csv
id,name,quantity,price
1,prod1,15,150
2,prod2,10,300
3,prod3,23,456
4,prod4,13,45
5,prod5,145,36
6,prod6,12,34
7,prod7,5,99
8,prod8,19,39
9,prod9,45,900

Go to rails console and run command

rake import

Finally check your products table, you will see the csv file data in to table.