This gist illustrates the flatten array method from ground up. :)

custom_array.rb

# This solution attempts to handle flattening of nested arrays gracefully
class CustomArray
  def flatten(array)
    raise 'input is not an array' unless array?(array)
    return [] if array.empty?

    recursive_flatten(array)
  end

  private

  def array?(array_param)
    array_param.is_a?(Array)
  end

  def recursive_flatten(array, results = [])
    array.each do |element|
      if array?(element)
        recursive_flatten(element, results)
      else
        results << element
      end
    end
    results
  end
end

test_custom_array.rb

require 'minitest/autorun'
require_relative 'custom_array'
# Respective test to ensure correctness of code written
class CustomArrayTest < Minitest::Test
  describe 'custom_array' do
    def setup
      @custom_array = CustomArray.new
    end

    it 'can get flatten response on expected inputs' do
      expected_response = [1, 2, 3, 5, 6, 3, 6, 9, 7, 9, 0]
      response = @custom_array.flatten([1, 2, 3, [5, 6, [3, 6, 9], 7, 9, 0]])

      assert_equal expected_response, response
    end

    it 'raises an error on non-array inputs' do
      assert_raises 'input is not an array' do
        @custom_array.flatten('')
      end
    end

    it 'returns empty arrays on empty array inputs' do
      assert_equal [], @custom_array.flatten([])
    end
  end
end