Just in case you ever find yourself needing to generate spreadsheet column identifiers from an arbitrary index

sheet_column_identifiers.rb

class SheetColumnIdentifiers
  def initialize
    @alphabet = ('A'..'Z').to_a
  end

  def [](index)
    return alphabet[index] if index < alphabet.length

    pad_index = (index / alphabet.length) - 1
    self[pad_index] + self[index % alphabet.length]
  end

  private

  attr_reader :alphabet
end

sheet_column_identifiers_spec.rb

RSpec.describe SheetColumnIdentifiers do
  context 'index less than 26' do
    it 'returns the alphabet character at given index' do
      expect(subject[0]).to eq 'A'
      expect(subject[25]).to eq 'Z'
    end
  end

  context 'index greater than 26, causing a two digit alpha ID' do
    it 'loops over itself, concatenating to create a spreadsheet row ID' do
      expect(subject[26]).to eq 'AA'
      expect(subject[28]).to eq 'AC'
    end
  end

  context 'index much greater than 26, causing first alpha digit to increment' do
    it 'loops over itself, concatenating to create a spreadsheet row ID' do
      expect(subject[52]).to eq 'BA'
      expect(subject[53]).to eq 'BB'
    end
  end

  context 'index so great that it requires 3 alpha digits' do
    it 'loops over itself, concatenating to create a spreadsheet row ID' do
      expect(subject[702]).to eq 'AAA'
      expect(subject[703]).to eq 'AAB'
    end
  end
end