_post_ids = []
_id = 1
def gen_document():
    global _post_ids, _id
    if _id == 1:
        doc_type, is_o2m = 'post', 0
    else:
        doc_type, is_o2m = choice([('post', 0), ('comment', 1), ('revision', 1)])
    
    doc = {'_id': str(_id), 'type': doc_type}
    
    if is_o2m:
        doc.update({'post_id': str(choice(_post_ids))})
    else:
        _post_ids.append(_id)
        
    _id += 1
    return doc