CODEWITHSUNDEEP
mongodbmongodb-query
peter.petrov
peter.petrov

Reputation: 39437

MongoDB - weird find behavior on embedded document

I have this collection in my test database.

[test] 2014-02-11 18:45:48.338 >>> db.blog.posts.find( );
{
        "_id" : ObjectId("52ee29bdb8eb94049427886a"),
        "comments" : [
                {
                        "author" : "Jane",
                        "comment" : "yes, yes, it is",
                        "votes" : 5
                }
        ],
        "content" : "This is a post",
        "id" : 5
}
{
        "_id" : ObjectId("52f95c5bd1fe9c171fe4344b"),
        "comments" : [
                {
                        "author" : "Jane",
                        "comment" : "yes, yes, it is",
                        "votes" : 5
                },
                {
                        "author" : "Nick",
                        "comment" : "test",
                        "votes" : 10
                },
                {
                        "author" : "John",
                        "comment" : "new one",
                        "votes" : 4
                }
        ],
        "content" : "This is a post",
        "id" : 5
}

When I run this find query:

[test] 2014-02-11 18:45:50.318 >>> db.blog.posts.find({ "comments" : {author : "Jane", comment: "yes, yes, it is", votes: 5}} );

I get both documents back.

But if I run this find query 

[test] 2014-02-11 18:46:18.428 >>> db.blog.posts.find({ "comments" : {author : "Jane", comment: "yes, yes, it is", votes: {$gte : 5}}} );

I get no documents back.

Why is that? How come == 5 gives me results back but >= 5 does not return anything.

Upvotes: 0

Views: 50

Answers (1)

Eugene P.
Eugene P.

Reputation: 2625

Very interesting case, to be honest. But, I think there
are more efficient commands to get what you need.

I have solved your issue as follows.

db.posts.find({
 "comments": {
    "$elemMatch": {
       author : "Jane", 
       comment: "yes, yes, it is", 
       votes: {"$gte" : 5}
    }
  }
});

and the result is as follows:

enter image description here

The reason the first OP query works and the second OP query doesn't is because
in the first case there are documents that meet the exact form being queried.
The second OP query uses a modifier, hence the need to wrap with $elemMatch.

More information from the official MongoDB site can be found here:
http://docs.mongodb.org/manual/reference/operator/projection/elemMatch/

Hope this helps.

Upvotes: 1

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