Set operation with condition - mongodb

Question

My object looks like:
{
_id: ObjectId(),
unpaid: 500,
paid: false
}
and the statement:
db.items.update({
unpaid: {
$gte: 500
}
}, {
$inc: {
unpaid: -500
}
});
When unpaid will be equal zero, the paid field needs also to change to true.
Is it possible to do this with one query?

Answer 1

Unfortunately, MongoDB queries are not flexible enough to trigger an post query event. You could do it in two ways, but both involve firing more than a single query:
The first approach, involves:
Finding and Updating all the records with unpaid >= 500 in a single
update statement.
Followed by another update statement which updates the paid field
to true for all unpaid = 0.
Code:
db.items.update({
"unpaid": {
$gte: 500
}
}, {
$inc: {
"unpaid": -500
}
});
db.items.update({
"unpaid":0,"paid":false
}, {
$set: {
"paid": true
}
});
This approach may not be suitable if atomic updates matter in your application, and could lead to adding extra app logic to see things correctly before the paid field has been updated.
The second approach, involves:
Finding all the records with unpaid >= 500.
Performing a bulk update. Each update will set both the unpaid and
the paid fields at the same time.
Code:
var bulk = db.items.initializeUnorderedBulkOp();
db.items.find({"unpaid":{$gte:500}}).forEach(function(doc){
var unpaid = doc.unpaid-500;
var update = {$set:{"unpaid":unpaid}};
if(unpaid <= 0){
update.$set["paid"] = true;
}
bulk.find({"_id":doc._id }).update(update);
})
bulk.execute();