Mongo query to find field within array of objects [duplicate]

Question

I have a collection called collectionA that contains an array of type objects.

Example:

{
"_id" : ObjectId("123456"),
"tags" : [ 
    {
        "name" : "Bob D",
    }, 
    {
        "name" : "another name",
    }
    ...

How would I write a query that matches the name within the array of objects? I've tried db.getCollection('collectionA').find({ "name": "Bob D}) but that didn't work.

Could you add sample document from your collection and expected output ? — mickl
– mickl, Commented Oct 3, 2018 at 15:48
Try db.getCollection('collectionA').find({ "tags.name": "Bob D" }) since name is nested inside tags — mickl
– mickl, Commented Oct 3, 2018 at 16:02
@mickl I've tried your command except I'm getting back the entire object (objectId(123456)) instead of the individual objects within the tags array that name matches to — Stackimus Prime
– Stackimus Prime, Commented Oct 3, 2018 at 16:17

mickl · Accepted Answer · 2018-10-03 16:29:15Z

2

To filter nested array you can either use $elemMatch (as projection):

db.col.find({ "tags.name": "Bob D" }, { tags: { $elemMatch: { name: "Bob D" } } })

which returns first matching array element, or use $filter to get multiple matching elements of tags array:

db.col.aggregate([ { $addFields: { tags: { $filter: { input: "$tags", as: "tag", cond: { $eq: [ "$$tag.name", "Bob D" ] } } } } } ])

answered Oct 3, 2018 at 16:29

mickl

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3 Comments

Stackimus Prime Over a year ago

Thanks a lot! Both methods work.

Stackimus Prime Over a year ago

Actually, to build off that, why do you need tags: { $elemMatch: { name: "Bob D" } in addition to db.col.find({ "tags.name": "Bob D" }? I know if I don't include elemMath, I get back the entire document instead of just the objects that matches with name.

mickl Over a year ago

Second argument of find() represents a projection which allows you to reshape the document you store in your db

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