MongoDB 2.4 MapReduce重构

Question

I recently upgraded to MongoDB 2.4 and as referenced in the release notes, am having an issue with a map function that makes use of db. The release notes recommend refactoring, but I am unclear as to what route I need to take.

The now non-working piece of the function is as follows:

function map() {
  var student = db.student.findOne(this.student_id);
  var school = db.school.findOne(this.school_id);
  ...
  emit({
    bcg_id: student.bcg_id,
  ...

I am unclear on how to pass the document from the 'student' collection to the 'student' variable now that db is deprecated.

Any recommendations?

Answer 1

You can no longer run a query inside a javascript function anymore.

See: http://docs.mongodb.org/manual/release-notes/2.4/#additional-limitations-for-map-reduce-and-where-operations

You can pass data into the scope eg:

res = t.mapReduce( mapper , reducer , { scope : { xx : 1 } } );

But as to how bigger a refactor - it depends on what the original map reduce function did.

You may want to collate all the data needed into a new collection and then run the map reduce over that instead.