Revamping MapReduce in MongoDB Version 2.4

After upgrading to MongoDB 2.4, I encountered an issue with a map function that uses db, as mentioned in the release notes. The release notes suggest refactoring, but I am unsure about the best approach to take.

The part of the function that is no longer working looks like this:

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 confused about how to assign the document from the 'student' collection to the 'student' variable now that db is deprecated.

Any advice on what steps to take?

javascriptmongodb

Answer №1

In Javascript functions, running a query is no longer possible.

For more information, please refer to: http://docs.mongodb.org/manual/release-notes/2.4/#additional-limitations-for-map-reduce-and-where-operations

If you need to pass data into the scope, you can do so using:

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

The extent of refactoring required will depend on the original map reduce function's purpose.

You might consider collecting all necessary data in a new collection and then performing the map reduce operation on that instead.

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