count_documents 给出错误：AttributeError：'dict'对象没有属性'_txn_read_preference'

Question

我编写了一个函数来检查行是否存在，如果不存在则插入，如果存在则更新。所以我尝试使用

count_documents

。我使用一个名为

filetype

的字段作为我的唯一密钥。

示例文档如下所示：

{
    "_id": ObjectId("652d47a64732d257ee31e846"),
    "filetype": "USTreasuryBillRates",
    "last_date_processed": "2023-10-16T00:00:00.000Z",
    "update_date": "2023-10-16T10:13:45.000Z"
}

Python函数：

def upsert_load_history(arg_file_type, arg_date, arg_db_collection):
    # insert if not there, otherwise update in place
    print("===== upsert_load_history ======")

    json_query = {"filetype": arg_file_type}
    row_count = arg_db_collection.count_documents(json_query, {'limit': 1})
    if row_count == 0:
        insert_load_history_new_rows(arg_file_type, arg_date, arg_db_collection)
    else:
        update_load_history_for_file_type(arg_file_type, arg_date, arg_db_collection)

错误：

===== upsert_load_history ======
Traceback (most recent call last):
  File "C:\GitHub\ClientName\Apps\LambdaUSTreasury\SetupLoadHistoryDB.py", line 134, in <module>
    upsert_load_history(file_type, file_date, db_collection_filehistory)
  File "C:\GitHub\ClientName\Apps\LambdaUSTreasury\SetupLoadHistoryDB.py", line 61, in upsert_load_history
    row_count = arg_db_collection.count_documents(json_query, {'limit': 1})
  File "C:\GitHub\ClientName\Apps\LambdaUSTreasury\pymongo\collection.py", line 1786, in count_documents
    _cmd, self._read_preference_for(session), session)
  File "C:\GitHub\ClientName\Apps\LambdaUSTreasury\pymongo\common.py", line 870, in _read_preference_for
    return session._txn_read_preference() or self.__read_preference
AttributeError: 'dict' object has no attribute '_txn_read_preference'

Answer 1

根据 JonRSharpe 的评论，这是我更正的代码。 基本上将 {'limit':1} 更改为 limit=1。

def upsert_load_history(arg_file_type, arg_date, arg_db_collection):
    # insert if not there, otherwise update in place
    print("===== upsert_load_history ======")
    update_date = datetime.datetime.now().strftime("%Y-%m-%dT%H:%M:%S.000Z")

    json_query = {"filetype": arg_file_type}
    row_count = arg_db_collection.count_documents(json_query, limit=1)
    if row_count == 0:
        insert_load_history_new_rows(arg_file_type, arg_date, arg_db_collection)
    else:
        update_load_history_for_file_type(arg_file_type, arg_date, arg_db_collection)

此外，根据 Belly Buster 的评论，我已经发现了“Upsert”功能。下面我编写了一个新方法来测试它，效果很好。

def update2_load_history_for_file_type(arg_file_type, arg_date, arg_db_collection):
    # do update without retrieve first, using the upsert feature
    print("===== update_load_history_for_file_type ======")

    # Note, in my collection, filetype is my unique key 
    json_query = {'filetype': arg_file_type}

    update_date = datetime.datetime.now().strftime("%Y-%m-%dT%H:%M:%S.000Z")
    json_update = {
         '$set': {
            'last_date_processed': arg_date,
            'update_date': update_date
         }
         }

    print("json_query:")
    pprint.pprint(json_query)
    print("json_update:")
    pprint.pprint(json_update)
    result = arg_db_collection.update_one(json_query, json_update, upsert=True)
    print("Update result.matched_count=", result.matched_count)