MongoDB aggreagte fill missing days [duplicate]

Question

I have a product collection with the following documents:

{ "_id" : 1, "item" : "abc", created: ISODate("2014-10-01T08:12:00Z") }
{ "_id" : 2, "item" : "jkl", created: ISODate("2014-10-02T09:13:00Z") }
{ "_id" : 3, "item" : "hjk", created: ISODate("2014-10-02T09:18:00Z") }
{ "_id" : 4, "item" : "sdf", created: ISODate("2014-10-07T09:14:00Z") }
{ "_id" : 5, "item" : "xyz", created: ISODate("2014-10-15T09:15:00Z") }
{ "_id" : 6, "item" : "iop", created: ISODate("2014-10-16T09:15:00Z") }

I want to draw a chart describing product count by day, so I use mongodb aggregation framework to count product group by day:

  var proj1 = {
      "$project": {
          "created": 1,
          "_id": 0,
          "h": {"$hour": "$created"},
          "m": {"$minute": "$created"},
          "s": {"$second": "$created"},
          "ml": {"$millisecond": "$created"}
      }
  };

  var proj2 = {
      "$project": {
          "created": {
              "$subtract": [
                  "$created", {
                      "$add": [
                          "$ml",
                          {"$multiply": ["$s", 1000]},
                          {"$multiply": ["$m", 60, 1000]},
                          {"$multiply": ["$h", 60, 60, 1000]}
                      ]
                  }]
          }
      }
  };

  db.product.aggregate([
          proj1,
          proj2,
          {$group: {
              _id: "$created",
              count: {$sum: 1}
          }},
          {$sort: {_id: 1}}
      ])

The result in mongo shell is:

{
    "result" : [ 
        {
            "_id" : ISODate("2014-10-01T00:00:00.000Z"),
            "count" : 1
        }, 
        {
            "_id" : ISODate("2014-10-02T00:00:00.000Z"),
            "count" : 2
        }, 
        {
            "_id" : ISODate("2014-10-07T00:00:00.000Z"),
            "count" : 1
        }, 
        {
            "_id" : ISODate("2014-10-15T00:00:00.000Z"),
            "count" : 1
        }, 
        {
            "_id" : ISODate("2014-10-16T00:00:00.000Z"),
            "count" : 1
        }
    ],
    "ok" : 1
}

Of course, there is no product some days and the chart using the result set above looks like this:

But the desired chart should look like this:

So the question is: How can I add missing days (of the last 30 days, for example) to the result set with count = 0? That means, the desired result set should looks like this:

{
    "result" : [
        {
            "_id" : ISODate("2014-09-16T00:00:00.000Z"),
            "count" : 0
        }, 
        {
            "_id" : ISODate("2014-09-17T00:00:00.000Z"),
            "count" : 0
        }, 
        ...            
        {
            "_id" : ISODate("2014-10-01T00:00:00.000Z"),
            "count" : 1
        }, 
        {
            "_id" : ISODate("2014-10-02T00:00:00.000Z"),
            "count" : 2
        }, 
        {
            "_id" : ISODate("2014-10-03T00:00:00.000Z"),
            "count" : 0
        },
        ...
        {
            "_id" : ISODate("2014-10-07T00:00:00.000Z"),
            "count" : 1
        }, 
        {
            "_id" : ISODate("2014-09-08T00:00:00.000Z"),
            "count" : 0
        },
        ...
        {
            "_id" : ISODate("2014-10-15T00:00:00.000Z"),
            "count" : 1
        }, 
        {
            "_id" : ISODate("2014-10-16T00:00:00.000Z"),
            "count" : 1
        },
        // also, add some extra days 
        {
            "_id" : ISODate("2014-10-17T00:00:00.000Z"),
            "count" : 0
        },
        {
            "_id" : ISODate("2014-10-10T00:00:00.000Z"),
            "count" : 0
        }
    ],
    "ok" : 1
}

Answer 1

Using aggregate to handle this question completely is a pain.
But it can be reached.
(MongoDB V2.6+ required)

var proj1 = {
    "$project" : {
        "created" : 1,
        "_id" : 0,
        "h" : {
            "$hour" : "$created"
        },
        "m" : {
            "$minute" : "$created"
        },
        "s" : {
            "$second" : "$created"
        },
        "ml" : {
            "$millisecond" : "$created"
        }
    }
};

var proj2 = {
    "$project" : {
        "created" : {
            "$subtract" : [ "$created", {
                "$add" : [ "$ml", {
                    "$multiply" : [ "$s", 1000 ]
                }, {
                    "$multiply" : [ "$m", 60, 1000 ]
                }, {
                    "$multiply" : [ "$h", 60, 60, 1000 ]
                } ]
            } ]
        }
    }
};

var group1 = {
        $group : {
            _id : "$created",
            count : {
                $sum : 1
            }
        }
    };

var group2 = {
        $group : {
            _id : 0,
            origin : {
                $push : "$$ROOT"
            },
            maxDate : {
                $max : "$_id"
            }
        }
};

var step = 24 * 60 * 60 * 1000; // milliseconds of one day

var project3 = {
    $project : {
        origin : 1,
        extents : {
            $map : {
                "input" : [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29],
                "as" : "e",
                "in" : {
                    _id : {
                        $subtract : [ "$maxDate", {
                            $multiply : [ step, "$$e"]
                        }]
                    },
                    count : {
                        $add : [0]
                    }
                }
            }
        }
    }
};

var project4 = {
        $project : {
            _id : 0,
            values : {
                $setUnion : [ "$origin", "$extents"]
            }
        }
};

var unwind1 = {
        $unwind : "$values"
};

var group3 = {
        $group : {
            _id : "$values._id",
            count : {
                $max : "$values.count"
            }
        }
};

db.product.aggregate([ proj1, proj2, group1, group2, project3, project4,
        unwind1, group3, {
            $sort : {
                _id : 1
            }
        } ]);

---

I would like to fill the missing part at application end something like this for your reference:

function sortResult(x, y) {
    var t1 = x._id.getTime();
    var t2 = y._id.getTime();
    if (t1 < t2) {
        return -1;
    } else if (t1 == t2) {
        return 0;
    } else {
        return 1;
    }
}

var result = db.product.aggregate();

var endDateMilliseconds = result[result.length - 1]._id.getTime();
var step = 24 * 60 * 60 * 1000; // milliseconds of one day

var map = {};
for (var i in result) {
    map[ result[i]._id.getTime() ] = result[i];
}

for (var ms = endDateMilliseconds, x = 1; x < 30; x++) {
    ms -= step;
    if ( ! ( ms in map ) ) {
        map[ms] = {_id : new Date(ms), count : 0};
    }
}

var finalResult = [];
for (var x in map) {
    finalResult.push(map[x]);
}
finalResult.sort(sortResult);
printjson(finalResult);

Answer 2

Ok, first of all: Non-existing values are evaluated to null (roughly translates to "nada", "nothing", "not there"), which isn't equal to 0, which is a well defined value.

MongoDB has no semantical understanding of the difference between 0 and 42, for example. So how should MongoDB decide which value to assume for a day in the time (of which mongo has no semantical understanding, too)?

Basically, you have two choices: save a 0 for each day when no value is to record or you iterate in your app over the days in the time you want to create a chart for and issue 0 for each day no value exists as a substitute. Id' suggest doing the former, since that would make it possible using the aggregation framework.