Update join in mongoDB. Is it possible? - mongodb

Question

Given these three documents:
db.test.save({"_id":1, "foo":"bar1", "xKey": "xVal1"});
db.test.save({"_id":2, "foo":"bar2", "xKey": "xVal2"});
db.test.save({"_id":3, "foo":"bar3", "xKey": "xVal3"});
And a separate array of information that references those documents:
[{"_id":1, "foo":"bar1Upd"},{"_id":2, "foo":"bar2Upd"}]
Is it possible to update "foo" on the two referenced documents (1 and 2) in a single operation?
I know I can loop through the array and do them one by one but I have thousands of documents, which means too many round trips to the server.
Many thanks for your thoughts.

Answer 1

It's not possible to update "foo" on the two referenced documents (1 and 2) in a single atomic operation as MongoDB has no such mechanism. However, seeing that you have a large collection, one option is to take advantage of the Bulk API which allows you to send your updates in batches instead of every update request to the server.
The process involves looping all matched documents within the array and process Bulk updates which will at least allow many operations to be sent in a single request with a singular response.
This gives you much better performance since you won't be sending every request to the server but just once in every 500 requests, thus making your updates more efficient and quicker.
-EDIT-
The reason of choosing a lower value is generally a controlled choice. As noted in the documentation there, MongoDB by default will send to the server in batches of 1000 operations at a time at maximum and there is no guarantee that makes sure that these default 1000 operations requests actually fit under the 16MB BSON limit. So you would still need to be on the "safe" side and impose a lower batch size that you can only effectively manage so that it totals less than the data limit in size when sending to the server.
Let's use an example to demonstrate the approaches above:
a) If using MongoDB v3.0 or below:
var bulk = db.test.initializeOrderedBulkOp(),
largeArray = [{"_id":1, "foo":"bar1Upd"},{"_id":2, "foo":"bar2Upd"}],
counter = 0;
largeArray.forEach(doc) {
bulk.find({ "_id": doc._id }).updateOne({ "$set": { "foo": doc.foo } });
counter++;
if (counter % 500 == 0) {
bulk.execute();
bulk = db.test.initializeOrderedBulkOp();
}
}
if (counter % 500 != 0 ) bulk.execute();
b) If using MongoDB v3.2.X or above (the new MongoDB version 3.2 has since deprecated the Bulk() API and provided a newer set of apis using bulkWrite()):
var largeArray = [{"_id":1, "foo":"bar1Upd"},{"_id":2, "foo":"bar2Upd"}],
bulkUpdateOps = [];
largeArray.forEach(function(doc){
bulkUpdateOps.push({
"updateOne": {
"filter": { "_id": doc._id },
"update": { "$set": { "foo": doc.foo } }
}
});
if (bulkUpdateOps.length === 500) {
db.test.bulkWrite(bulkUpdateOps);
bulkUpdateOps = [];
}
});
if (bulkUpdateOps.length > 0) db.test.bulkWrite(bulkUpdateOps);