Use a postgres string[] column for both singular entries and arrays. If the question type is a MULTISELECT, then the attribute will act like an array, but if not, it acts like a String. However, this particular String does not respond to <<, so that it will throw errors if you try to use it. This way, if the developer is has an array, and trying…

question.rb

class CreateQuizzes < ActiveRecord::Migration
  def change
    create_table :quiz_questions do |t|
      t.integer :quiz_id
      t.string :format, :default => 'MultiChoice' #MultiChoice, MultiSelect, or Field
      t.string :answer, array: true, default: []
      t.string :options, array: true, default: []
      t.timestamps
    end
  end
end


class QuizQuestion < ActiveRecord::Base
  
  MULTISELECT = 'MultiSelect'
  MULTICHOICE = 'MultiChoice'
  FIELD = 'Field'
  QUESTION_TYPES = [MULTISELECT, MULTICHOICE, FIELD]

  after_initialize -> { self[:answer] = []; self[:options] = []  }
    
  def answer
    if(format == MULTISELECT)
      # array answer
      self[:answer]
    else
      #There should only be one answer, String
      res = self[:answer].first
      unless res.nil?
        class << res
          undef_method :<<
        end
      end
      return res
    end
  end

  def answer=(other)
    if(format == MULTISELECT)
      if(other.is_a?(Array))
        self[:answer] = other
      else
        raise ArgumentError, ':answer for MultiChoice is an array, use `answer<<(String)` to add an item, or `answer=Array` to set to a new set'
      end
    else
      #There should only be one answer
      self[:answer][0] = other
    end
  end

end