This method allow you to charge documents you already have
        in your database. In this way an Index would be created according to
        the model and fields registered.

def charge_documents(self):
    """
    This method allow you to charge documents you already have
        in your database. In this way an Index would be created according to
        the model and fields registered.
    """

    doc_count   = self._whoosh.doc_count()
    objs        = orm.count(e for e in self._model)

    field_names = set(self._schema_attrs.keys())
    missings    = set(self._whoosh.schema.names())

    for f in list(field_names - missings):
        self.add_field(f, fields.TEXT(self.kw))

    if doc_count == 0 and objs > 0:
      writer = self._whoosh.writer()
      for obj in orm.select(e for e in self._model):
        attrs = {self._primary_key: obj.get_pk()}
        for f in list(self._schema_attrs.keys()):
          attrs[f] = str(getattr(obj, f))
        writer.add_document(**attrs)
      writer.commit()