func (file *GridFile) SetId(id interface{}) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.Id = id
	file.m.Unlock()
}
func (file *GridFile) SetId(id interface{}) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.Id = id
	file.m.Unlock()
}
// SetId changes the current file Id.
//
// It is a runtime error to call this function once the file has started
// being written to, or when the file is not open for writing.