Mongo $filter (aggregation) get only one element of array

Question

I do some filter with mongodb but it return all of the data in my array. But i only want to get the specific element from that array. I cant find it in the document.

db.sales.aggregate([
       {
          $project: {
             items: {
                $filter: {
                   input: "$items",
                   as: "item",
                   cond: { $gte: [ "$$item.price", 100 ] }
                }
             }
          }
       }
    ])

Run above command I will this is result

{
   "_id" : 0,
   "items" : [
      { "item_id" : 2, "quantity" : 1, "price" : 240 }
   ]
}

Question is I only want to get the price

  {
       "_id" : 0,
       "items" : [
          { "price" : 240 }
       ]
    }

or even

{
    "price" : 240 
}

How to do it?

Possible duplicate of How to filter and map array of documents in MongoDB query? — s7vr
– s7vr, Commented Dec 13, 2017 at 3:19

Pavan Vora · Accepted Answer · 2017-12-13 04:03:51Z

2

You actually need $map to "alter" the array elements returned, as $filter only "selects" the array elements that "match". Try to run the below code.

ds.sales.aggregate([
    {
        $project: {
            items: {
                $map: {
                    input: {
                        $filter: {
                            input: "$items",
                            as: "item",
                            cond: { $gte: [ "$$item.price", 100 ] }
                         }
                    },
                    "as": "a",
                    "in": {
                        "price": "$$a.price"
                      }
                }
            }
        }
    }], function (err, list) {
    ...

answered Dec 13, 2017 at 4:03

Pavan Vora

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Comments

Tony Chou · Accepted Answer · 2017-12-13 03:33:33Z

I don't know your whole data looks like, if your data looks like this

{
    "_id" : 0,
    "items" : [
        {
            "item_id" : 1,
            "quantity" : 5,
            "price" : 80
        },
        {
            "item_id" : 2,
            "quantity" : 1,
            "price" : 240
        },
        {
            "item_id" : 3,
            "quantity" : 4,
            "price" : 320
        }
    ]
}

Just try this

> db.sales.aggregate([
    {'$unwind': '$items'},
    {'$project': {'price': '$items.price'}},
    {'$match' : {'price': {'$gte': 100 }}}
])
{ "_id" : 0, "price" : 240 }
{ "_id" : 0, "price" : 320 }

$unwind

{'items': [{'item_id': 1}, {'item_id': 2}]}

after $unwind

{'items': {'item_id': 1}}
{'items': {'item_id': 2}}

$project

This can choose which field you want ( or just remove which field you don't want) and rename a field to what you want.

{'items': {'item_id': 1}}

after $project

{'renamefor__item_id': 1}

$match

Just see the previous link for more detail. My English is not very good:(

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Mongo $filter (aggregation) get only one element of array

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$unwind

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$match

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