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wb = xlsx_package.workbook
wb.add_worksheet(name: "Student") do |sheet|
  title = wb.styles.add_style(b: true, bg_color: "395870", fg_color: "FFFFFF",
                              alignment: {horizontal: :center})
  sheet.add_row ["ID", "Name", "Date of Birth", "Email",
                               "School", "School Address"], style: title
  @students.each do |student|
    sheet.add_row [student.id, student.full_name, student.DOB, student.email,
                   student.school_name, student.school_address]
  end

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[...]

def index
  @students = Student.all_with_school_details

  respond_to do |format|
    format.html
    format.csv { send_data @students.as_csv }
    format.xlsx
  end

105ron

source 'https://rubygems.org'

# Use axlsx to create spreadsheets
gem 'rubyzip', '~> 1.1.0'
gem 'axlsx', '2.1.0.pre'
gem 'axlsx_rails'

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[...]

<%= link_to "Export CSV", students_path(format: :csv) %>

[...]

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def index
  @students = Student.all_with_school_details

  respond_to do |format|
    format.html
    format.csv { send_data @students.as_csv }
end

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students GET  /students(.:format) students#index

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[...]

def self.as_csv
  CSV.generate do |csv|
    columns = %w(id first_name last_name DOB school_name school_address)
    csv << columns.map(&:humanize)
    all_with_school_details.each do |student|
      csv << student.attributes.values_at(*columns)
    end
  end

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Student.all_with_school_details.first.school_name

=> "Springfield Elementary School"

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Student.all_with_school_details.first

=> #<Student:0x007f9c26baa580
 id: 1,
 first_name: "Bart",
 last_name: "Simpson",
 DOB: Sun, 17 Dec 1989,
 email: "bart@the-simpsons.com",
 created_at: Sun, 04 Dec 2016 06:26:52 UTC +00:00,
 updated_at: Sun, 04 Dec 2016 06:26:52 UTC +00:00,

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require 'csv'

class Student < ApplicationRecord
  
  belongs_to :school
  
  [...]
  
  def self.all_with_school_details
    Student.select("students.*, schools.name as school_name, schools.address as school_address").joins(:school)