MongoDB course m0101 homework 2-2 for node using mapReduce

hw2-2.js

var mongodb = require('mongodb');
var _ = require('underscore');

var db = mongodb.Db('students', new mongodb.Server('localhost', 27017), {safe: true});

db.open(function(err, client) {
	if(err) {
		console.log('Error opening connection' + err);
	}
	console.log('db opened');
	var Grades = new mongodb.Collection(client, 'grades');
	console.log('Grades collection');
	Grades.count(function(err, cnt) {
        if (err) {
	        console.log(err);
	    }
	    console.log('There are ' + cnt 
                + ' records in Grades');
    }); // count result
	var qry = {"type":"homework"};
    var mapf = function() {
            emit( this.student_id,
                  {
                      student_id: this.student_id,
                      lowGrade: this.score,
                      docid: this._id
                  });
    };
    var reducef = function(key, values) {
            var r = {
                      student_id: key,
                      lowGrade: null,
                      docid: null
                    }      
            var lg = null, di = null; 
            values.forEach(function(v) {
                    if ( v.lowGrade < lg || lg == null) {
                            lg = v.lowGrade;
                            di = v.docid;
                    }   
            });
            r.lowGrade = lg;
            r.docid = di;
            return r;
    };
    Grades.mapReduce(mapf, reducef, 
        {
            query: qry,
            out: { reduce: "lowest_grades" },
        },
        function(err, rsltColl) { 
	        if (err) {
		        console.log(err);
	        }  
	        console.log('mapReduce callback...');
	        rsltColl.count(function(err, cnt) {
    	        if (err) {
		            console.log(err);
	            }
	            console.log('There are ' + cnt 
                        + ' records in lowest_grades');
            	rsltColl.find().toArray(function(err, delDocs) {
            		if (err) {
            			console.log(err);
            		}  
            		console.log('start looping results...');
            		var nbDel = 0;
            		_.each(delDocs, function(iter) {
            		        nbDel++;    
            		        console.log(nbDel+": "+iter.value.docid);
            		        Grades.remove({"_id":iter.value.docid}, {safe:true}, function(err, nbRemoved) {
                    	        if (err) {
                    		        console.log(err);
                    	        }                   
                	            console.log(nbRemoved + ' records removed');
            		        }) // remove
            		}); // each delDocs
                 	client.close();
            		console.log('done removing '+nbDel+' records');
            	}); // find delDocs
            }); // count result
            
        } // mapReduce callback
    ); // mapReduce()
}); // db.open()

remove in finalize().js

var mongodb = require('mongodb');
var _ = require('underscore');

var db = mongodb.Db('students', new mongodb.Server('localhost', 27017), {safe: true});

db.open(function(err, client) {
	if(err) {
		console.log('Error opening connection' + err);
	}
	console.log('db opened');
	var Grades = new mongodb.Collection(client, 'grades');
	console.log('Grades collection');

	// count Grades before mapReduce (should be 800)
	Grades.count(function(err, cnt) {
        if (err) {
	        console.log(err);
	    }
	    console.log('There are ' + cnt 
                + ' records in Grades');
    }); // count Grades before mapReduce
    
    // set up the stuff for mapReduce:
    // only act on documents with type=homework
	var qry = {"type":"homework"};
	
	// per student map the score and the _id of the document
    var mapf = function() {
            emit( this.student_id,
                  {
                      student_id: this.student_id,
                      lowGrade: this.score,
                      docid: this._id
                  });
    };
    
    // reduce this to the lowest score per student
    var reducef = function(key, values) {
            var r = {
                      student_id: key,
                      lowGrade: null,
                      docid: null
                    }      
            var lg = null, di = null; 
            values.forEach(function(v) {
                    if ( v.lowGrade < lg || lg == null) {
                            lg = v.lowGrade;
                            di = v.docid;
                    }   
            });
            r.lowGrade = lg;
            r.docid = di;
            return r;
    };
    
    // now remove this document with the lowest scrore from Grades

    // THIS DOES *NOT* WORK, MONGO DOES NOT ALLOW IT

    var finalizef = function(key, value) {
        	Grades.remove({"_id":value.docid}, {safe:true}, function(err, nbRemoved) {
                if (err) {
                 console.log(err);
                }                   
            });
    };
    
    // call it
    Grades.mapReduce(mapf, reducef,
        {
            query: qry,
            out: { reduce: "lowest_grades" },
            finalize: finalizef
        },
        function(err, rsltColl) { 
	        if (err) {
		        console.log(err);
	        }  
	        console.log('mapReduce callback...');
	        // count the result (should be 200)
	        rsltColl.count(function(err, cnt) {
    	        if (err) {
		            console.log(err);
	            }
	            console.log('There are ' + cnt 
                        + ' records in lowest_grades');
    	        // count the remaining grades (should be 600)
            	Grades.count(function(err, cnt) {
            		if (err) {
            			console.log(err);
            		}  
	                console.log('There are now' + cnt 
                            + ' records in Grades');
            	}); // count Grades after
            }); // count result
            
        } // mapReduce callback
    ); // mapReduce()
}); // db.open()