MONGO & Node.js: How to get an object within an array?

Question

I just wonder how I can get an object within an array of objects. I just need that object.

This is the collection:

{
    "net" : "192.168.1.1/24",
    "_id" : ObjectId("531d1c2d857831021c48e3af"),
    "ips" : [ 
        {
            "ips" : "192.168.1.1",
            "_id" : ObjectId("531d1c2d857831021c48e3b3")
        }, 
        {
            "ips" : "192.168.1.33",
            "_id" : ObjectId("531d1c2d857831021c48e3b2")
        }, 
        {
            "ips" : "192.168.1.38",
            "_id" : ObjectId("531d1c2d857831021c48e3b1")
        }, 
        {
            "ips" : "192.168.1.106",
            "_id" : ObjectId("531d1c2d857831021c48e3b0")
        }
    ],
    "__v" : 0
}

I need the object with "531d1c2d857831021c48e3b0" ID, what is the sentence to get only that object? I tried with db.nets.find({ "ips._id": ObjectId("531d1c2d857831021c48e3b3") } ) but I get the whole collection.

Thank you very much.

Edit. With the code I receive this:

db.nets.find(
      { 
     "_id": ObjectId("531d1c2d857831021c48e3af"),
     "ips._id": ObjectId("531d1c2d857831021c48e3b3") 
 },
 { "ips.$" : 1 }
 )

{
    "_id" : ObjectId("531d1c2d857831021c48e3af"),
    "ips" : [ 
        {}, 
        {}, 
        {}, 
        {}
    ]
}

but what I want to receive is this:

{
            "ips" : "192.168.1.33",
            "_id" : ObjectId("531d1c2d857831021c48e3b2")
        }

Your added response is not possible in current versions. What version of MongoDB are you running? Or did you actually do a find or findOne without specifying the primary _id of the document. The answer makes that point more clear now with a slight edit. — Neil Lunn
– Neil Lunn, Commented Mar 13, 2014 at 13:31
I'm using MongoDB version: 2.0.6. I checked it in console writing "mongo". — Daniel 976034
– Daniel 976034, Commented Mar 13, 2014 at 14:21
yeaaaaah!!!!!! It was the version!!! Now I have 2.4.9 version and it works perfectly. Thank you very munch!! — Daniel 976034
– Daniel 976034, Commented Mar 13, 2014 at 14:34

Neil Lunn · Accepted Answer · 2014-03-13 13:31:28Z

1

Use projection. The positional $ operator allows you to just select the matching index of the array.

 db.nets.find(
     { 
         "_id": ObjectId("531d1c2d857831021c48e3af"),
         "ips._id": ObjectId("531d1c2d857831021c48e3b3") 
     },
     { "ips.$" : 1 }
 )

Which gives this response:

{
    "_id" : ObjectId("531d1c2d857831021c48e3af"),
    "ips" : [
            {
                    "ips" : "192.168.1.1",
                    "_id" : ObjectId("531d1c2d857831021c48e3b3")
            }
    ]
}

It's not the "whole collection", it's the "whole document", in which unless you filter the fields with a "projection" you will get by default.

edited Mar 13, 2014 at 13:31

answered Mar 13, 2014 at 11:09

Neil Lunn

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4 Comments

Neil Lunn Over a year ago

@user2976034 I think you just tried to post a multi-line response in a comment. It didn't work. But of course I do test things so, yes this will only return one array element. Which is what you are asking for are you not?

Daniel 976034 Over a year ago

Hello Neil. Sorry, I sent the last comment without writing completely. I wanted to write: Thank you for your explanation about $ operator, I didn't know it (I'm a complete newbie). With the code you wrote, I don't receive what I want. I think I don't explain correctly. Check the first post to see what I receive and the object that I want to receive if it's possible. Thank you very much for your time.

Neil Lunn Over a year ago

@user2976034 as I explained. I think your result is because you did not filter the document that actually has this array member in it. You need to do that.

Daniel 976034 Over a year ago

Hi again Neil, I did a screenshot because it gives to me other result than you! oi59.tinypic.com/34dhfyt.jpg

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