MongoEngine - Atomic Updates

Atomicity is one of the ACID transaction properties. A database transaction has to be inspanisible and irreducible so that it either occurs completely or doesn’t occur at all. This property is called Atomicity. MongoDB supports Atomicity only on single documents and not on multi-document transactions.

MongoEngine provides the following methods for atomic updates on a queryset.

update_one() − Overwrites or adds first document matched by query.

update() − Performs atomic update on fields matched by query.

modify() − Update a document and return it.

Following modifiers may be used with these methods. (These modifiers come before the field, not after).

The following is an example of atomic update, we first create a Document class called tests and add a document in it.

from mongoengine import *
con=connect( newdb )

class tests (Document):
   name=StringField()
   attempts=IntField()
   scores=ListField(IntField())

t1=tests()
t1.name= XYZ 
t1.attempts=0
t1.scores=[]
t1.save()

Let us use update_one() method to update name field from XYZ to MongoDB.

tests.objects(name= XYZ ).update_one(set__name= MongoDB )

The push modifier is used to add data in ListField (scores).

tests.objects(name= MongoDB ).update_one(push__scores=50)

To increment attempts field by one, we can use inc modifier.

tests.objects(name= MongoDB ).update_one(inc__attempts=1)

The updated document looks as follows −

{
"_id":{"$oid":"5ebcf8d353a48858e01ced04"},
"name":"MongoDB",
"attempts":{"$numberInt":"1"},
"scores":[{"$numberInt":"50"}]
}