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

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.

Also worth mentioning the projection of the subdocument after the query db.<myCollection>.find({ "makes.fgh": { "$exists": true } }, { "makes.fgh": 1 })
It's starting to make more sense now, thanks both for your help!
A gotcha for the node driver - projection param should have "projection" as a key, e.g. db.<myCollection>.find({ "makes.fgh": { "$exists": true } }, { projection: { "makes.fgh": 1 }})

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