00_value_object.rb

class Leg
  include ActiveModel::Validations
  include ActiveModel::Naming
  include ActiveModel::Conversion
  include Virtus

  attribute :start_day, Integer
  attribute :end_day,   Integer
end

v1_model.rb

# without Rails `serialize` macro, using our own getters/setters and serializeation

validate { legs.all?(&:valid?) }

# set default for column
after_initialize do
  write_attribute(:legs, JSON.dump([])) if read_attribute(:legs).blank?    
end

# getter
def legs
  @legs ||= JSON.parse(read_attribute(:legs)).map { |attrs| Leg.new(attrs) }
end
  
# setter
def legs=(_legs)
  @legs = _legs
end

# setter
# @presenter
def legs_attributes=(legs_attributes)
  self.legs = legs_attributes.map { |attrs| Leg.new(attrs) }
  write_attribute(:legs, JSON.dump(@legs))
end

v2_model.rb

# using Rails `serialize` macro.

validate { legs.valid? }

serialize :legs, Legs

# Legs acts as a serializer for an array of Leg objects
class Legs
  include ActiveModel::Validations
  include ActiveModel::Naming
  include ActiveModel::Conversion
  include Virtus

  attribute :legs, Array[Leg]

  validate :legs_valid

  def initialize(legs_attributes = [])
    super(:legs => legs_attributes)
  end

  # delegate everything to the Array, so we don't have to do model.legs.legs.each
  def method_missing(method_name, *args, &block)
    legs.send(method_name, *args, &block) if legs.respond_to?(method_name)
  end

  private

  def legs_valid
    legs.all?(&:valid?)
  end
end