person.rb

require 'json'
class Person
  @age = nil
  attr_accessor :json_field, :age

  def initialize *args
    # useful for Rails ApplicationRecord models
    args[0].each{|key, value| self.send("#{key}=".to_sym, value)}
    # uncomment next line for inherited classes
    # super *args
  end

  # catch missing methods
  def method_missing name, *args
    # @davidbella's inspiration: https://gist.github.com/davidbella/6918455
    # define json accessor to read/write data in json_field
    attribute_name = name.to_s.gsub(/=$/, '')
    self.json_field ||= {}
    # define charles.{attribute_name} = value
    self.class.send(:define_method, "#{attribute_name}=".to_sym) do |value|
      self.json_field.send(:[]=, attribute_name, value)
    end
    # define charles.{attribute_name}
    self.class.send(:define_method, attribute_name.to_sym) do
      value = self.json_field.send(:[], attribute_name)
    end
    # call defined method
    send(name, *args)
  end

  # Just for object inspection
  def to_h
    Hash[instance_variables.map{|var| [var, instance_variable_get(var)] }]
  end

end

# Initialize instance and assign dynamic and static attributes
charles = Person.new({first_name: 'Charles', age: 35})
#
# Verify dynamic attribute
puts charles.first_name.inspect
# -> "Charles"
#
# Assign more dynamic variables and check json field
charles.country, charles.city = 'UK', 'London'
puts JSON.pretty_generate(charles.json_field)
# -> {
#      "first_name": "Charles",
#      "country": "UK",
#      "city": "London"
#    }
#
# Inspect instance attributes
puts JSON.pretty_generate(charles.to_h)
# -> {
#      "@json_field": {
#        "first_name": "Charles",
#        "country": "UK",
#        "city": "London"
#      },
#      "@age": 35
#    }
#
# For ApplicationRecord, remember to notfify changes before save
# charles.json_field_will_change!
# charles.save