How I imagine the migration stuff happening

old_node.rb

class OldNode

  def lesson
    @lesson ||= Lesson.find(id)
  end
  
  def verify_migration
    self.verify_field_names
    self.verify_sections
  end
  
  private
  
  def verify_field_names
    raise unless lesson.title    ==  self.title
    raise unless lesson.sections ==  self.sections
  end

  def verify_sections
    lesson.sections.each_with_index do |section, index|
      raise unless section[:title] == self.sections[index][:title]
    end
  end  
end

old_node_spec.rb

describe OldNode
  before(:each) do
    # somewhere you generate an oldNode. Either form a factory,
    # inline (a la OldNode.new(...) or from an import.
    @old_node = TommyMagic.generate(:node)
  end
  it "should verify the field names" do
    old_node.migrate
    expect{old_node.verify_migration}.to_not raise_error
  end
end

Oh! You might want to create models I haven't. E.g. a section model and a parts model.