为何 PyMongo 在记录中插入 _id 字段

MongoDB 要求每个文档都必须有 _id 
如果待插入的文档没有 _id, MongoDB 会自动生成一个,但不会把结果返回个 PyMongo
对于需要写操作频繁的应用来说, 在写入之前复制一份插入 _id 代价可能会很高

如果你不想自动生成的 _id, 就需要自己在插入数据前 自己准备好 _id 字段

Why does PyMongo add an _id field to all of my documents?¶

When a document is inserted to MongoDB using 

insert_one()

, 

insert_many()

,
or 

bulk_write()

,
and that document does not include an 

_id

 field, PyMongo automatically adds one for
you, set to an instance of 

ObjectId

.
For example:

>>> my_doc = {'x': 1}
>>> collection.insert_one(my_doc)
<pymongo.results.InsertOneResult object at 0x7f3fc25bd640>
>>> my_doc
{'x': 1, '_id': ObjectId('560db337fba522189f171720')}

Users often discover this behavior when calling 

insert_many()

 with
a list of references to a single document raises 

BulkWriteError

.
Several Python idioms lead to this pitfall:

>>> doc = {}
>>> collection.insert_many(doc for _ in range(10))
Traceback (most recent call last):
...
pymongo.errors.BulkWriteError: batch op errors occurred
>>> doc
{'_id': ObjectId('560f171cfba52279f0b0da0c')}

>>> docs = [{}]
>>> collection.insert_many(docs * 10)
Traceback (most recent call last):
...
pymongo.errors.BulkWriteError: batch op errors occurred
>>> docs
[{'_id': ObjectId('560f1933fba52279f0b0da0e')}]

PyMongo adds an 

_id

 field in this
manner for a few reasons:
All MongoDB documents are required to have an 

_id

 field.
If PyMongo were to insert a document without an 

_id

 MongoDB
would add one itself, but it would not report the value back to PyMongo.
Copying the document to insert before adding the 

_id

 field
would be prohibitively expensive for most high write volume applications.
If you don’t want PyMongo to add an 

_id

 to
your documents, insert only documents that already have an 

_id

 field, added by your
application.