Selecting an Object in Array

Question

I want to select an object within an array with id condition. In the below img i want to select [0].

enter image description here

db.users.find({"injury._id": ObjectId("537233061845d2ec10000071")})

Thankx for Help.

Can I please implore you to stop posting screenshots. They are horrible to try and read and data ( which you have at least posted some text this time ) is better represented as text so people can actually use it — Neil Lunn
– Neil Lunn, Commented May 21, 2014 at 10:04
With that out of the way. What is your actual question? Do you want to select an array element with an _id value or do you want to select the index 0 of an array? And then do what? — Neil Lunn
– Neil Lunn, Commented May 21, 2014 at 10:05
You are with the query you show above. You mean something else perhaps? Please clarify what you mean by editing your question. — Neil Lunn
– Neil Lunn, Commented May 21, 2014 at 10:07

Neil Lunn · Accepted Answer · 2014-05-21 10:17:55Z

2

I'm guessing here that you want to only see the selected array element in the document you are searching for. Use projection:

db.users.find(
    { "injury._id": ObjectId("537233061845d2ec10000071")},
    { "injury.$": 1 }
)

That just returns the matching element rather than all of them.

If you expect more than one match ( unlikely with an ObjectId, but for future reference ) then use the .aggregate() method instead:

db.users.aggregate([

    // Match documents first, helps reduce the pipeline
    { "$match": {
       "injury._id": ObjectId("537233061845d2ec10000071")
    }},

    // Unwind the array
    { "$unwind": "$injury" },

    // Actually match the elements
    { "$match": {
       "injury._id": ObjectId("537233061845d2ec10000071")
    }},

    // Group back if you really want arrays
    { "$group": {
        "_id": "$_id",
        "injury": { "$push": "$injury" },
        // and use "$first" for any other fields
    }}
])

So now you know what to do when you come up against that one as well.

edited May 21, 2014 at 10:17

answered May 21, 2014 at 10:11

Neil Lunn

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2 Comments

Anup Over a year ago

Thankx...Works good...next time i will try to post it without screenshots..:)

Neil Lunn Over a year ago

@Anup Thankyou. And there you also have the basic example included in the answer for when you need to filter more than one result from the array.

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