def document(self, document):
        """
        Associate a :class:`~elasticsearch_dsl.Document` subclass with an index.
        This means that, when this index is created, it will contain the
        mappings for the ``Document``. If the ``Document`` class doesn't have a
        default index yet (by defining ``class Index``), this instance will be
        used. Can be used as a decorator::

            i = Index('blog')

            @i.document
            class Post(Document):
                title = Text()

            # create the index, including Post mappings
            i.create()

            # .search() will now return a Search object that will return
            # properly deserialized Post instances
            s = i.search()
        """
        self._doc_types.append(document)

        # If the document index does not have any name, that means the user
        # did not set any index already to the document.
        # So set this index as document index
        if document._index._name is None:
            document._index = self

        return document