MongoDB Aggregate $unwind $match using date - what did I miss?

Question

I'm new in MongoDB and I'm trying to work with aggregations. I partially do what I'm looking for but I have a strange behavior with dates.

MongoDB info

Version : 2.2.0

Operating System : Windows 7

Objective

Get all comments created after '2012-11-22'

Let's get an example :

Data

db.blogs.save([ {
    title : "X this is my second title",
    author : "max",
    posted : new Date(),
    pageViews : 10,
    tags : [ "good", "nice" ],
    comments : [ {
        "_id" : ObjectId("50ac9fdb53a900bcb4be46d9"),
        author : "john",
        text : "pretty awesome",
        create : ISODate("2012-12-20T00:00:00.000Z")
    }, {
        "_id" : ObjectId("50ac9fd003a900bcb4be46d9"),
        author : "sam",
        text : "this is bad",
        create : ISODate("2012-12-22T00:00:00.000Z")
    } ],
    other : {
        foo : 5
    }
}, {
    title : "X this is my title",
    author : "bob",
    posted : new Date(),
    pageViews : 5,
    tags : [ "fun", "good", "fun" ],
    comments : [ {
        "_id" : ObjectId("50ac55db53a900bcb4be46d9"),
        author : "matthieu",
        text : "bof bof",
        create : ISODate("2012-12-21T00:00:00.000Z")
    }, {
        "_id" : ObjectId("50ac55db53a900bcb4b226d9"),
        author : "sam",
        text : "this s bad",
        create : ISODate("2012-12-22T00:00:00.000Z")
    } ],
    other : {
        foo : 6
    }
}, {
    title : "X NEW ELEMENT",
    author : "emil",
    posted : new Date(),
    pageViews : 33,
    tags : [ "bad", "hehe", "cool", "nice" ],
    comments : [ {
        "_id" : ObjectId("50ac55db531100bcb4b226d9"),
        author : "emilie",
        text : "could be better",
        create : ISODate("2012-12-21T00:00:00.000Z")
    }, {
        "_id" : ObjectId("50ac55db101100bcb4b226d9"),
        author : "samuel",
        text : "maybe a good one",
        create : ISODate("2012-12-20T00:00:00.000Z")
    } ],
    other : {
        foo : 9
    }
}, {
    title : "X Y NEW ELEMENT",
    author : "marc",
    posted : new Date(),
    pageViews : 33,
    tags : [ "bad", "hehe", "cool", "nice" ],
    comments : [ {
        "_id" : ObjectId("50ac55db101100bcb4baa6d9"),
        author : "sam",
        text : "hehe",
        create : ISODate("2012-11-20T00:00:00.000Z")
    }, {
        "_id" : ObjectId("50ac55db101ab0bcb4baa6d9"),
        author : "daniel",
        text : "yeehhhh hoho",
        create : ISODate("2012-11-23T00:00:00.000Z")
    } ],
    other : {
        foo : 9
    }
} ])

---

Example 1 : OK with strings matching

Return all 'comments' from user 'sam' :

db.blogs.aggregate( [
   { $unwind: "$comments" },
   { $match: { 'comments.author' : "sam" } },
   { $group: { _id: "$comments" } }
] )

This return only comments where property 'author' is 'sam'.

---

Example 2 : issue with dates ?

this aggregation is (for me) the same as the previous one but instead of matching 'author', I match the date property 'create' :

db.blogs.aggregate( [
   { $unwind: "$comments" },
   { $match: { 
    'comments.create' : {
        $gt: ISODate("2012-11-22T00:00:00Z")
    }
   } },
   { $group: { _id: "$comments" } }
] )

But if you test this aggregation, you will see that some comments contains 'create' dates lower than '2012-11-22'. For instance, comment with ID '50ac9fdb53a900bcb4be46d9' is returned.

---

I would expect only comments with dates greater than '2012-11-22'... I guess I missed something...

Thank you

Answer 1

Ho my god! Stennie is right. It's November and not December...

If I put 2012-12-21T00:00:00Z it's working... ^^

Btw, as JohnnyHK said it's maybe better to do the operation this way :

db.blogs.aggregate( [
    { $project : { 'comments' : 1 } },
    { $unwind: "$comments" },
    { 
       $match: { 
          'comments.create' : {
                $gt: ISODate("2012-12-21T00:00:00Z")
          }
       } 
    }
])

Without using $group but using $project it seems that I get what I'm looking for.

Thank you very much both for your feedbacks!