```ruby
require 'perpetuity/mapper'

module Perpetuity
  describe Mapper do
    let(:mapper) do
      Mapper.new do
      end
    end

    subject { mapper }

    it { should be_a Mapper }

    it 'has correct attributes' do
      Mapper.new { attribute :name, String }.attributes.should eq [:name]
    end

    it 'returns an empty attribute list when no attributes have been assigned' do
      Mapper.new.attributes.should be_empty
    end

    it 'can have embedded attributes' do
      mapper_with_embedded_attrs = Mapper.new { attribute :comments, Array, embedded: true }
      mapper_with_embedded_attrs.attribute_set[:comments].should be_embedded
    end

    its(:mapped_class) { should eq Object }

    context 'with unserializable attributes' do
      let(:serialized_attrs) do
        [ Marshal.dump(Comment.new) ]
      end

      it 'serializes attributes' do
        article = Article.new
        article.comments = [Comment.new]
        mapper.attributes_for(article)[:comments].should eq serialized_attrs
      end

      describe 'unserializes attributes' do
        let(:comments) { mapper.unserialize(serialized_attrs)  }
        subject { comments.first }

        it { should be_a Comment }
        its(:body) { should eq 'Body' }
      end
    end
  end
end
```

Fix specs broken by removing test_classes require
user
```ruby
require 'perpetuity/mapper'

module Perpetuity
  describe Mapper do
    let(:mapper) do
      Mapper.new do
      end
    end

    subject { mapper }

    it { should be_a Mapper }

    it 'has correct attributes' do
      Mapper.new { attribute :name, String }.attributes.should eq [:name]
    end

    it 'returns an empty attribute list when no attributes have been assigned' do
      Mapper.new.attributes.should be_empty
    end

    it 'can have embedded attributes' do
      mapper_with_embedded_attrs = Mapper.new { attribute :comments, Array, embedded: true }
      mapper_with_embedded_attrs.attribute_set[:comments].should be_embedded
    end

    its(:mapped_class) { should eq Object }

    context 'with unserializable attributes' do
      let(:unserializable_object) { 1.to_c }
      let(:serialized_attrs) do
        [ Marshal.dump(unserializable_object) ]
      end

      it 'serializes attributes' do
        object = Object.new
        object.stub(sub_objects: [unserializable_object])
        mapper.attribute :sub_objects, Array, embedded: true
        mapper.attributes_for(object)[:sub_objects].should eq serialized_attrs
      end

      describe 'unserializes attributes' do
        let(:comments) { mapper.unserialize(serialized_attrs)  }
        subject { comments.first }

        it { should be_a Complex }
        it { should eq unserializable_object }
      end
    end
  end
end
```
assistant