Get specific fields from mongodb and return as object array

Question

I have the following document:

"content": [
    {
        "_id": "5dbef12ae3976a2775851bfb",
        "name": "Item AAA",
        "itemImages": [
            {                 
                "_id": "5dbef12ae3976a2775851bfd",
                "imagePath": "https://via.placeholder.com/300",
                "imageTitle": "Test 300"
            },
            {
                "_id": "5dbef12ae3976a2775851bfc",
                "imagePath": "https://via.placeholder.com/250",
                "imageTitle": "Test 250"                 
            }
        ]
    }

and I am wondering if there is a way to return only the data in the array yet with the "name" and document "main _id" so that result set will be

        "itemImages": [
            {                 
                "_id": "5dbef12ae3976a2775851bfb",
                "name": "Item AAA",                    
                "imagePath": "https://via.placeholder.com/300",
                "imageTitle": "Test 300"
            },
            {
                "_id": "5dbef12ae3976a2775851bfb",
                "name": "Item AAA",                    
                "imagePath": "https://via.placeholder.com/250",
                "imageTitle": "Test 250"                 
            }
        ]

I've tried using mongodb find and aggregate functions but neither helped in retrieving the above results. Thanks for your help

Joe · Accepted Answer · 2019-11-06 00:11:12Z

1

You should be able to get what you want with aggregation.

You'll need to:

unwind the array so there is a separate document for each element
use addFields to add the _id and name to the element
group to reassemble the array
project to get just the field you want

This might look something like:

db.collection.aggregate([
     {$unwind:"$itemImages"},
     {$addFields: {
           "itemImages._id":"$_id",
           "itemImages.name":"$name"
     }},
     {$group:{_id:"$_id", itemImages:{$push:"$itemImages"}}},
     {$project:{itemImages:1,_id:0}}
])

answered Nov 6, 2019 at 0:11

Joe

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2 Comments

Mahmoud Metwally Over a year ago

Thanks Joe for your help, worked perfectly, one more question if you don't mind please. I am trying to slice the result to return images from X number of items using { $project: { itemImages: { $slice: [ '$itemImages', offset, size ] }, _id: 0 } } yet by applying the $slice command it slice the number of images returned in a single item rather than number of items returned, so was wondering what i am doing wrong / missing here? Thanks again

Joe Over a year ago

Remember that the $project is applied separately to each document in the stream. If you want to limit the number of documents returned, use $limit and/or $skip. Also note that if you do not include a sort, paginating with skip/limit may not return what you expect. Also, since aggregate returns a cursor, iterating using cursor methods is usually more efficient.

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