In a previous article we exported data from a Rails database to XLSX using a gem.

We can export to CSV without any external gems, because Ruby has an in-built CSV processor.

export-csv-rails

I see two good approaches to generating CSV:

1. *.csv.erb template without Ruby::CSV (elementary approach) #

A rails app can respond to format csv by default.

We can create a template with some data formatted as CSV lines and have a link to download the rendered page.

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  def index
    @users = User.all
    respond_to do |format|
      format.html
      format.csv
    end
  end

# app/views/users/index.hmtl.erb
<%= link_to "CSV export", users_path(format: :csv), download: ['Users', Date.today].join(' ') %>

# app/views/users/index.csv.erb

# send all fields
<% fields = [:id, :email, :last_name] %>
<%= fields.map(&:to_s).join(";") %>
<% @users.each do |user| %>
<%= fields.map { |field| user[field].to_s }.join(";") %>
<% end %>

# or send selected fields
<%= User.column_names.map(&:to_sym).join(";") %>
<% @users.each do |user| %>
<%= user.attributes.values_at(*User.column_names).join(";") %>
<% end %>

This approach requires quite a lot of data transformation.

2. *.csv.erb template with Ruby::CSV (more correct approach) #

Instead of transforming data in the above template, we can let Ruby::CSV handle the generation of correctly formatted CSV lines.

# app/views/users/index.hmtl.erb
<%= link_to 'Export Users', users_path(format: :csv) %>

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  require 'csv'

  def index
    @users = User.all
    respond_to do |format|
      format.html
      format.csv do
        filename = ['Users', Date.today].join(' ')
        response.headers['Content-Type'] = 'text/csv'
        response.headers['Content-Disposition'] = "attachment; filename=#{filename}.csv"
        render template: 'users/index'
      end
    end
  end

# app/views/users/index.csv.erb
<%- headers = ['Last Name', 'Email'] -%>
<%= CSV.generate_line headers %>
<%- @users.each do |user| -%>
  <%= CSV.generate_line([user.last_name, user.email]) -%>
<%- end -%>

This approach is more pure/less hacky.

3. Without a template (more generic approach) #

Use [send_file method](https://api.rubyonrails.org/v6.1.4/classes/ActionController/DataStreaming.html] to assign user attributes that should be present in the generated CSV:

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  require 'csv'

  def index
    @users = User.all
    respond_to do |format|
      format.html
      format.csv do
        filename = ['Users', Date.today].join(' ')
        send_data User.to_csv(@users), filename:, content_type: 'text/csv'
      end
    end
  end

Add to_csv method to the model

# app/models/user.rb
class User < ApplicationRecord
  def self.to_csv(collection)
    CSV.generate(col_sep: ';') do |csv|
      csv << column_names
      collection.find_each do |record|
        csv << record.attributes.values
      end
    end
  end
end

I think this is a great approach to export all records and their attributes without messing with templates.

4. Concern: (most generic approach) #

To make the #3 approach more scalable, we can extract the to_csv into a model convern that can be shared across different models:

module GenerateCsv
  extend ActiveSupport::Concern
  require 'csv'

  class_methods do
    def to_csv(collection)
      CSV.generate(col_sep: ';') do |csv|
        # csv << attribute_names
        csv << column_names

        collection.find_each do |record|
          csv << record.attributes.values
        end
      end
    end
  end
end

# app/models/user.rb
class User < ApplicationRecord
  include GenerateCsv
end

That’s it!