Mimicking Transactional Behavior - MongoDB Applied Design Patterns - page 31

someone must have removed that line item and we must try to $push it onto the array with its
new quantity:

def def increase_qty ( order_id , sku , price , qty ):

total_update = price * qty
while while True :

result = db . orders . update (
{ '_id' : order_id , 'items.sku' : sku },
{ '$inc' : {
'total' : total_update ,
'items.$.qty' : qty } })
iif result [ 'updatedExisting' ]: break break

result = db . orders . update (
{ '_id' : order_id , 'items.sku' : { '$ne' : sku } },
{ '$inc' : { 'total' : 110.22 },
'$push' : { 'items' : { 'sku' : sku ,
'qty' : qty ,
'price' : price } } })
iif result [ 'updatedExisting' ]: break break

Optimistic Update with Compensation

There are some cases where it's just not possible to do your operation with a single update()
statement in MongoDB. For instance, consider the account transfer problem where we must
debitoneaccount andcredit another.Inthesecases,wearestuckmakingmultiple updates,but
we must ensure that our database is eventually consistent by examining all the places where
we could have an error. A naive approach would simply store the account balance in each doc-
ument and update them separately. Our documents would be quite simple:

{ _id : 1 , balance : 100 }
{ _id : 2 , balance : 0 }

The code to update them is likewise simple:

def def transfer ( amt , source , destination ):

result = db . accounts . update (
{ '_id' : source , 'balance' : { '$gte' : amt } },
{ '$inc' : { 'balance' : - amt } })
iif not

not result [ 'updatedExisting' ]:
raise

raise InsufficientFundsError ( source )