Simple class to easily build CSV outputs for your POROs or ActiveRecord models.

csv_builder.rb

# CsvBuilder configures a model to generate a resource. Inspired by ActiveAdmin.
#
# Usage example:
#
#   csv_builder = CSVBuilder.new User, col_sep: ';'
#   csv_builder.column :id
#   csv_builder.column :email
#   csv_builder.column("Name") { |resource| resource.display_name }
#   csv_builder.headers %w[ID Email Name]
#   csv_builder.scope { |scope| scope.where('created_at > ?', 7.days.ago) } # unavailable for POROs
#   csv_builder.generate
#
class CsvBuilder
  attr_reader :columns, :options

  def initialize(klass, options = {})
    fail ArgumentError, 'Must provide a Class' unless klass.is_a?(Class)
    @klass = klass
    @options = options.reverse_merge(row_sep: "\n")
    @columns = []
    @headers = []
  end

  def column(str, &block)
    columns << Column.new(str, block)
  end

  def header(str)
    @headers << str
  end

  def headers(arr)
    @headers = arr
  end

  def scope(&block)
    if @klass.ancestors.include?(ActiveRecord::Base) # naive approach
      @scope = block if block.is_a?(Proc)
    else
      raise StandardError.new(@klass.to_s + ' is not a decendant of ActiveRecord::Base')
    end
  end

  def generate(io = '')
    io << CSV.generate_line(generate_header_row, @options)
    resources.each do |resource|
      io << CSV.generate_line(generate_data_row(resource), @options)
    end
    io
  end

  class Column
    attr_reader :name, :data

    def initialize(str, block = nil)
      @name = str
      @data = block || str.to_sym
    end
  end

  private

  def generate_header_row
    if @headers.empty?
      columns.map do |column|
        if column.name.is_a?(Symbol)
          @klass.human_attribute_name(column.name)
        else
          column.name
        end
      end
    else
      @headers
    end
  end

  def generate_data_row(resource)
    columns.map do |column|
      if column.data.is_a?(Proc)
        column.data.call resource
      else
        resource.public_send(column.data)
      end
    end
  end

  def resources
    if @scope
      @scope.call(@klass).find_each.lazy
    else
      @klass.all.find_each.lazy
    end
  end
end