CODEWITHSUNDEEP
mongodbmongodb-query
mhk
mhk

Reputation: 424

Two Same MongoDB queries one yeild results one empty responce

db.posts.find({comments:{$elemMatch:{user:'Khan'}}})
db.posts.find({comments:{$elemMatch:{user:'khan'}}})

this is really a mind twister for me, above are two $elemMatch search queries i use in MongoDb shell, i typed these in Code Studio, when i copy paste first line in shell it return nothing, when i copy paste the 2nd line in shell, expected result is returned... both are same then why the response is different??? shell screenshot following is the posts collection in the db

{
    "_id": {
        "$oid": "5eda19c6a38d4418e311e43b"
    },
    "title": "post1",
    "body": "body of post 1",
    "category": "news",
    "tags": [
        "news",
        "events"
    ],
    "user": {
        "name": "author name",
        "status": "author"
    },
    "date": "Fri Jun 05 2020 15:09:10 GMT+0500 (Pakistan Standard Time)",
    "likes": 6,
    "comments": [
        {
            "user": "mehtab",
            "body": "this is some comments 1",
            "date": "Fri Jun 05 2020 16:47:17 GMT+0500 (Pakistan Standard Time)"
        },
        {
            "user": "khan",
            "body": "this is some comments 2",
            "date": "Fri Jun 05 2020 16:47:17 GMT+0500 (Pakistan Standard Time)"
        },
        {
            "user": "saad",
            "body": "this is some comments 3",
            "date": "Fri Jun 05 2020 16:47:17 GMT+0500 (Pakistan Standard Time)"
        }
    ]
}

MongoDB 4.2.7 Community

Upvotes: 1

Views: 33

Answers (2)

David Makogon
David Makogon

Reputation: 71120

Your two queries are indeed different. One has the first letter upper-cased, and one does not, and string-matching is case-sensitive.

Uppercase K:

db.posts.find({comments:{$elemMatch:{user:'Khan'}}})

Lowercase k:

db.posts.find({comments:{$elemMatch:{user:'khan'}}})

Upvotes: 2

GM-atteoni
GM-atteoni

Reputation: 455

I'm sure you want to know why this happens and me too, but following the MongoDB Documentation, if you specify a single query predicate in the $elemMatch expression, $elemMatch is not necessary.

Since the $elemMatch only specifies a single condition, the $elemMatch expression is not necessary, and instead you can use the following query:

db.posts.find(
   { "comments.user": "Khan" }
)

Upvotes: 1

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