MongoDB find key on nested object KEY (JSON)

Question

So pretty new and absolutely uneducated on MongoDB.

Having this JSON structure:

{
"id": "123456",
"makes": {
    "abc": {
        "att1": 4,
        "att2": "fffff",
        "att3": 46
        },
    "fgh": {
        "att1": 8,
        "att2": "ggggg",
        "att3": 6
    },
    "rty": {
        "att1": 3,
        "att2": "hhhh",
        "att3": 4
        },
    "iop": {
        "att1": 4,
        "att2": "llll",
        "att3": 3
        }
}

}

how can I query the DB for "fgh" make? I tried:

db.<myCollection>.find({"makes":"fgh"})

but that doesn't work. It works fine if I write:

db.<myCollection>.find({"makes.fgh.att1":8})

thank you in advance!

mtj · Accepted Answer · 2016-08-18 13:11:20Z

26

When you try to query for makes.fgh, you don't do a query on the content, but on the structure, as "fgh" is not a value but a sub-document.

You can achieve this with a $exists search:

db.myCollection.find( { "makes.fgh" : { $exists : true } })

See https://docs.mongodb.com/manual/reference/operator/query/exists/ for reference.

To integrate @chridam's helpful comment:

If you are only interested in that sub-document, you can also add a projection to the find:

db.myCollection.find({ "makes.fgh" : { $exists : true }}, { "makes.fgh" : 1 })

Have a look at https://docs.mongodb.com/manual/reference/method/db.collection.find/#db.collection.find for details.

edited Aug 18, 2016 at 13:11

answered Aug 18, 2016 at 13:02

mtj

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6 Comments

chridam Over a year ago

Also worth mentioning the projection of the subdocument after the query db.<myCollection>.find({ "makes.fgh": { "$exists": true } }, { "makes.fgh": 1 })

mtj Over a year ago

Yes, right. Edited the answer to include this fact.

Zenigata Over a year ago

It's starting to make more sense now, thanks both for your help!

danwild Over a year ago

A gotcha for the node driver - projection param should have "projection" as a key, e.g. db.<myCollection>.find({ "makes.fgh": { "$exists": true } }, { projection: { "makes.fgh": 1 }})

openCivilisation Over a year ago

Would it be possible to somehow pattern match? eg: fg*?

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