Question

I am currently trying to do a simple mapreduce over some documents stored in MongoDB. I use

map = BSON::Code.new "function() { emit(this.userid, 1); }"

for the mapping and

reduce = BSON::Code.new "function(key, values) {
   var sum = 0;
   values.forEach(function(value) {
    sum += value;
   });
   return sum;
}"

for the reduction. This works fine when I call map_reduce the following way:

output = col.map_reduce(map, reduce, # col is the collection in mongodb, e.g. db.users
            {
                :out => {:inline => true},
                :raw => true
            }
          )

Now to the real question: How can I use the upper call to map_reduce to enable sorting? The manual says, that I must use sort and an array of [key, direction] pairs. I guessed the following should work, but it doesn’t:

output = col.map_reduce(map, reduce,
            {
                :sort => [["value", Mongo::ASCENDING]],
                :out => {:inline => true},
                :raw => true
            }
          )

Do I have to choose another datatype? The option also doesn’t work (same error), when using an empty [], although the manual says that is the default for the option. Unfortunately the error message from MongoDB doesn’t help too much:

/usr/lib/ruby/gems/1.9.1/gems/mongo-1.3.1/lib/mongo/db.rb:506:in `command': Database command 'mapreduce' failed: {"assertion"=>"sort has to be blank or an Object", "assertionCode"=>13609, "errmsg"=>"db assertion failure", "ok"=>0.0} (Mongo::OperationFailure)
    from /usr/lib/ruby/gems/1.9.1/gems/mongo-1.3.1/lib/mongo/collection.rb:576:in `map_reduce'
    from ./mapreduce.rb:26:in `<main>'

If you need the full runnable code, please say so in the comments. I exclude it for now as it only contains the initialization of a connection to mongodb and initialization of the collection col by querying a database.

Answer 1

Use a BSON::OrderedHash and it will work.

output = col.map_reduce(map, reduce,
    {
         :sort => BSON::OrderedHash.new[{"value", Mongo::ASCENDING}],
         :out => {:inline => true},
         :raw => true
    }
)