Demo on how Active Record in Rails works

demo_schema.rb

SCHEMA = {
  users: [:id, :email, :name]
}

class MiniRecord
  def self.columns
    table = (self.name.downcase + "s").to_sym
    SCHEMA[table]
  end

  # Callback invoked whenever a subclass of the current class is created.
  # Ref: https://apidock.com/ruby/Class/inherited
  def self.inherited(subclass)
    cols = subclass.columns || []
    cols.each do |col|
      # Defines an instance method in the receiver.
      # Ref: https://apidock.com/ruby/Module/define_method
      subclass.define_method(col) do
        @attributes ||= {}
        @attributes[col]
      end

      subclass.define_method("#{col}=") do |value|
        @attributes ||= {}
        @attributes[col] = value
      end
    end
  end

  def self.before_save(method_name)
    @before_save_callbacks ||= []
    @before_save_callbacks << method_name
  end

  def self.before_save_callbacks
    @before_save_callbacks || []
  end

  def save
    self.class.before_save_callbacks.each do |cb|
	    # Invokes the method identified by symbol, passing it any arguments specified.
	    # Ref: https://apidock.com/ruby/Object/send
      send(cb)
    end
    puts "💾 Saving to database..."
    puts "   Data: #{@attributes.inspect}"
  end
end

class User < MiniRecord
  before_save :normalize_email

  def normalize_email
    self.email = email.downcase if email
    puts "🔧 before_save: normalize_email"
  end
end

u = User.new
u.email = "demo@rails.com"
u.name = "Mini Rails"
puts u.email
puts u.name
u.save