Lets suppose we want to import a products CSV file in rails i.e. reading rows from CSV file and inserting its into a table. There are the following steps which we will use to import CSV file.

Create an application using command

rails new csv_import
cd csv_import

Modify Gemfile to using MYSQL RDBMS

Path: csv_import/Gemfile

gem 'mysql2'
#and run 
bundle install 

Now configure your app for database

Path: csv_import/config/database.yml

default: &default
  adapter: mysql2
  encoding: utf8
  host: localhost
  template: template0
  database: csv_import 
  pool: 5
  username: xxxx
  password: xxxx

development:
  <<: *default
  database: csv_import

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  <<: *default
  database: test_csv_import

production:
  <<: *default
  database: prod_csv_import

Here , I've chosen adapter: mysql2 , because of using of gem ' mysql2' and create db by command

rake db:create

This command create db named csv_import .

Now, modify application.rb  to add csv class

Path: csv_import/config/application.rb

require File.expand_path('../boot', __FILE__)

require 'rails/all'
require 'csv'

Bundler.require(*Rails.groups)

module CsvImport
  class Application < Rails::Application
   
    config.active_record.raise_in_transactional_callbacks = true
  end
end

Now we'll create a model to import/upload and display the products data

rails g model Product prod_name:string quantity:integer price:float

It'll generate the file like..

Running via Spring preloader in process 18795
      invoke  active_record
      create    db/migrate/20160704061419_create_products.rb
      create    app/models/product.rb
      invoke    test_unit
      create      test/models/product_test.rb
      create      test/fixtures/products.yml

And run command to create table

rake db:migrate

Its give the output like...

== 20160704061419 CreateProducts: migrating ===================================
-- create_table(:products)
   -> 0.0633s
== 20160704061419 CreateProducts: migrated (0.0634s) ==========================

Here we'll generate interface to to upload a CSV file

rails g controller Products index import

The ouput look likes

Running via Spring preloader in process 19253
      create  app/controllers/products_controller.rb
       route  get 'products/import'
       route  get 'products/index'
      invoke  erb
      create    app/views/products
      create    app/views/products/index.html.erb
      create    app/views/products/import.html.erb
      invoke  test_unit
      create    test/controllers/products_controller_test.rb
      invoke  helper
      create    app/helpers/products_helper.rb
      invoke    test_unit
      invoke  assets
      invoke    coffee
      create      app/assets/javascripts/products.coffee
      invoke    scss
      create      app/assets/stylesheets/products.scss

This command creates views and routes for index and import.

Now modify routes file like

Path: csv_import/config/routes.rb

Rails.application.routes.draw do

  resources :products do
    collection { post :import }
  end

  root to: "products#index"

end

Now, lets we modify product model to accept the CSV and perform import functionality and modify it to like...

path: csv_import/app/models/product.rb

class Product < ActiveRecord::Base
  require 'csv'
  def self.import(file)
    CSV.foreach(file.path, 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.except("price"))
      else
        Product.create!(product_hash)
      end
    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.

Now we will edit product controller and add the import & index action to call this function.

class ProductsController < ApplicationController

  def index
    @products = Product.all
  end

  def import
    begin
      Product.import(params[:file])
      redirect_to root_url, notice: "Products successfully imported."
    rescue
      redirect_to root_url, notice: "Invalid CSV file format."
    end
  end

end

When we will hit url "http://localhost:3000/"

The index page open with product list and a form by which user can upload new products to db.