def index
  @reports = Report.preload(:categories)
end

csv.items = @reports

# When there are many records
# csv.items = @reports.find_each

# When there are many records with decorator
# csv.items = @reports.find_each.lazy.map(&:decorate)

# csv.filename = "reports_#{Time.current.to_i}.csv"
# csv.streaming = false

csv.cols.add('Update date') { |r| l(r.updated_at.to_date) }
csv.cols.add('Categories') { |r| r.categories.pluck(:name).join(' ') }
csv.cols.add('Content', :content)
csv.cols.add('Empty')
csv.cols.add('Static', 'dummy')

csv = Csb::Builder.new(items: items)
csv.cols.add('Update date') { |r| l(r.updated_at.to_date) }
csv.cols.add('Categories') { |r| r.categories.pluck(:name).join(' ') }
csv.cols.add('Content', :content)
csv.cols.add('Empty')
csv.cols.add('Static', 'dummy')
csv.build

# =>
# Update date,Categories,Content,Empty,Static
# 2019/06/01,category1 category2,content1,,dummy
# 2019/06/02,category3,content2,,dummy

# Your view
csv.items = @articles
csv.cols = Article.csb_cols

# Your Model
def self.csb_cols
  Csb::Cols.new do |cols|
    cols.add('Update date') { |r| I18n.l(r.updated_at.to_date) }
    cols.add('Categories') { |r| r.categories.pluck(:name).join(' ') }
    cols.add('Title', :title)
  end
end

# Your test
require 'csb/testing'

expect(Article.csb_cols.col_pairs(article)).to eq [
  ['Update date', '2020-01-01'],
  ['Categories', 'test rspec'],
  ['Title', 'Testing'],
]

expect(Article.csb_cols.as_table(articles)).to eq [
  ['Update date', 'Categories', 'Title'],
  ['2020-01-01', 'test rspec', 'Testing'],
  ['2020-02-01', 'rails gem', 'Rails 6.2'],
]

gem 'csb'

Csb.configure do |config|
  config.utf8_bom = true # default: false
  config.streaming = false # default: true
  config.after_streaming_error = ->(error) do # default: nil
    Rails.logger.error(error)
    Bugsnag.notify(error)
  end
end