'Drops the entire collection associated with this
:class:`~mongoengine.Document` type from the database.
Raises :class:`OperationError` if the document has no collection set
(i.g. if it is `abstract`)
.. versionchanged:: 0.10.7
:class:`OperationError` exception raised if no collection available'

@classmethod
def drop_collection(cls):

    col_name = cls._get_collection_name()
    if (not col_name):
        raise OperationError(('Document    %s    has    no    collection    defined    (is    it    abstract    ?)' % cls))
    cls._collection = None
    db = cls._get_db()
    db.drop_collection(col_name)
