old_contents: 
  from admin.nodes.serializers import serialize_node
  
  def serialize_preprint(preprint):
  
      return {
          'id': preprint._id,
          'date_created': preprint.date_created,
          'modified': preprint.date_modified,
          'provider': preprint.provider,
          'node': serialize_node(preprint.node),
          'is_published': preprint.is_published,
          'date_published': preprint.date_published,
          'subjects': preprint.subjects,
      }
  
new_contents: from website.project.taxonomies import Subject
  from admin.nodes.serializers import serialize_node
  
  
  def serialize_preprint(preprint):
  
      return {
          'id': preprint._id,
          'date_created': preprint.date_created,
          'modified': preprint.date_modified,
          'provider': preprint.provider,
          'node': serialize_node(preprint.node),
          'is_published': preprint.is_published,
          'date_published': preprint.date_published,
          'subjects': serialize_subjects(preprint.subjects),
      }
  
  
  def serialize_subjects(subjects):
      serialized_subjects = []
      for subject in subjects:
          subject = Subject.load(subject[0])
          serialized_subjects.append({
              'id': subject._id,
              'text': subject.text
          })
      return serialized_subjects
  
subject: add subject serializing method