python - 为什么 pymongo 在对不同对象使用 insert_one 时会生成重复的 _id 值？

Question

我正在尝试使用 pymongo 将对象保存到 mongodb。我对第一个对象没有任何问题，但是在尝试保存第二个对象时我得到了 pymongo.errors.DuplicateKeyError: E11000 duplicate key error collection: km_tracker.entries index: _id_ dup key: { : ObjectId('5b8ce80ebb822e06c8ecf1c7') }

我的保存功能:

def save_entries(entries):
    entries['save_date'] = str(datetime.datetime.now())
    db.entries.insert_one(entries)

回溯:

Traceback (most recent call last):
  File "app.py", line 182, in <module>
    main();
  File "app.py", line 22, in main
    new_entry()
  File "app.py", line 77, in new_entry
    review_information(entries)
  File "app.py", line 178, in review_information
    save_entries(entries)
  File "app.py", line 93, in save_entries
    db.entries.insert_one(entries)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\collection.py", line 693, in insert_one
    session=session),
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\collection.py", line 607, in _insert
    bypass_doc_val, session)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\collection.py", line 595, in _insert_one
    acknowledged, _insert_command, session)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\mongo_client.py", line 1243, in _retryable_write
    return self._retry_with_session(retryable, func, s, None)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\mongo_client.py", line 1196, in _retry_with_session
    return func(session, sock_info, retryable)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\collection.py", line 592, in _insert_command
    _check_write_command_response(result)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\helpers.py", line 217, in _check_write_command_response
    _raise_last_write_error(write_errors)
  File "C:\Users\someuser\AppData\Local\Programs\Python\Python37\lib\site-packages\pymongo\helpers.py", line 198, in _raise_last_write_error
    raise DuplicateKeyError(error.get("errmsg"), 11000, error)
pymongo.errors.DuplicateKeyError: E11000 duplicate key error collection: km_tracker.entries index: _id_ dup key: { : ObjectId('5b8ce6adbb822e40d431d444') }

第一个成功存入数据库的对象:

{
"_id" : ObjectId("5b8ce6adbb822e40d431d444"),
"reg_number" : "dfg",
"date" : "dfg",
"b_meter_indication" : "dfg",
"end_meter_indication" : "dfg",
"trip" : "dfg",
"start_address" : "dfg",
"stop_address" : "dfg",
"reason" : "dfg",
"driver" : "dfg",
"other" : "dfg",
"save_date" : "2018-09-03 09:45:49.340871"
}

第二个对象，由于重复键没有保存:

{'_id': ObjectId('5b8ce6adbb822e40d431d444'),
 'b_meter_indication': 'rty',
 'date': 'rty',
 'driver': 'rty',
 'end_meter_indication': 'rty',
 'other': 'rty',
 'reason': 'rty',
 'reg_number': 'try',
 'save_date': '2018-09-03 09:46:02.246101',
 'start_address': 'rty',
 'stop_address': 'rty',
 'trip': 'rty'
}

因为我没有明确定义 _id 的值, 但让 pymongo 为我做这个，我不明白为什么它会分配以前的值 _id到我当前的对象。在这种情况下，出于某种原因，pymongo 会不会认为第二个对象与第一个对象相同，从而给它相同的 _id值(value)？

Python 版本:3.7.0MongoDB 版本 4.0PyMongo 版本:3.7.1

编辑:添加了使用 save_entries() 函数的函数

def edit_entry(entries):
    print("Editing: {}".format(entries))
    entry = input()
    return entry

def review_information(entries):
    print("Do you wish to edit something? (y/n)")
    while edit != False or edit != False:
        edit = input()

        if edit == "Y" or edit == "y":
            edit_entry(entries)
        elif edit == "N" or edit == "n":
            break
        else:
            print("Please provide a valid input")
            continue
    save_entries(entries)

Answer 1

I'm not explicitly defining the value of _id, but letting pymongo do this for me

这是因为当使用 insert_one() 将文档插入 MongoDB 时, insert_many() , 或 bulk_write() ，并且该文档不包含 _id 字段，PyMongo自动为您添加一个，设置为 ObjectId 的实例。

另见 FAQ: Why does PyMongo add an _id field to all of my documents?

I have no issues with the first object, but when trying to save a second object I get pymongo.errors.DuplicateKeyError

Python 没有显式按值传递和按引用传递语义，而是按名称传递值。列表和字典是可变对象，数字、字符串和元组是不可变对象(immutable对象)。

在您的情况下，本质上您是将相同的 entries 对象传递给 save_entries() 函数。 PyMongo insert_one() 通过添加 _id 字段修改了对象，然后您再次尝试保存该字段。然而，第二次保存已经包含 _id 字段，导致重复错误。

有几种方法可以处理这种情况:

• 明确 copy()传递给 save_entries() 之前的对象。
• 在每次调用 save_entries() 后从对象中删除 _id 键。即 del entries["_id"]。