Deletes all the  documents using the  pk associated to them.

def delete_documents(self):
    """Deletes all the  documents using the  pk associated to them.
    """
    pk = str(self._primary_key)
    for doc in self._whoosh.searcher().documents():
      if pk in doc:
        doc_pk = str(doc[pk])
        self._whoosh.delete_by_term(pk, doc_pk)