Question

Is it true that Map/Reduce results can be stored permanently, but not sorted? For example,

coll = Analytic.collection.map_reduce(map, reduce, 
                                        :out => 'analyticsCachedResult')

the above is stored permanently but is not sorted.

To sort it on the fly, it can be

coll.find({}, :sort => ['value.pageviews', :desc]) 

But can you sort the table internally? I think if an index is created for the sort key, then it will be almost as good as sorting it internally?

Or, how can we sort it, and save it as a new collection analyticsSortedCachedResult?

I think later on, to get any collection using Mongoid, it is by

coll = Mongoid.master.collection('analyticsCachedResult')

? but I am just worried that since Mongoid.master.collection('foo') will still return a collection even if the collection doesn’t exist. But looks like it is not saved into MongoDB either.

Answer 1

MapReduce in MongoDB outputs to a regular collection, so you can use all the features you are used to, including sorting or even doing another MapReduce pass on the output. You may have issues with sorting a large collection without an index on the sort key though.