debug map/reduce, How to get size exceeding document?

Question

I am running a map/reduce job and it fails because the reduce step exceeds the 16 mb limit for bson documents. And I run out of ideas how to debug it.

exception: BSONObj size: 16808001 (0x41780001) is invalid.
Size must be between 0 and 16793600(16MB)
First element: 0: { 531.343: { 2: [ { strength: 121.0, acc: 23.0 } ] } }

My problem is that I don't got access to the document but only the first element of the input. Is there a way to get the complete thing?

My idea was to check the size of the output within the reduce javascript and print the data into the mongo log. But

1. I found no way to determine the BSON size of something within a map/reduce function
2. I tried to estimate the size BSON with JSON but it never triggered:

if(JSON.stringify(ret).length > 10000000) {
    print('Document of size: ' + JSON.stringify(ret).length);
    print(JSON.stringify(ret));
    print();
}
return ret;

Any other ideas?

Answer 1

Object.bsonsize({ ...... }) will calculate the BSON size of the document.

Also 10000000/1024/1024 is only ~ 9mb so I'm not sure your test would have caught it.