MongoDB get size of unwinded array

Question

I'm trying to return size of 'orders' and sum of 'item' values for each 'order' for each order from documents like the example document:

    orders: [
    {
        order_id: 1,
        items: [
            {
                item_id: 1,
                value:100
            },
            {
                item_id: 2,
                value:200
            }
        ]
    },
    {
        order_id: 2,
        items: [
            {
                item_id: 3,
                value:300
            },
            {
                item_id: 4,
                value:400
            }
        ]
    }
]

I'm using following aggregation to return them, everything works fine except I can't get size of 'orders' array because after unwind, 'orders' array is turned into an object and I can't call $size on it since it is an object now.

db.users.aggregate([
{
    $unwind: "$orders"
},
{
    $project: {
        _id: 0,
        total_values: {
            $reduce: {
                input: "$orders.items",
                initialValue: 0,
                in: { $add: ["$$value", "$$this.value"] }
            }
        },
        order_count: {$size: '$orders'}, //I get 'The argument to $size must be an array, but was of type: object' error
    }
},
])

the result I expected is:

{order_count:2, total_values:1000} //For example document
{order_count:3, total_values:1500} 
{order_count:5, total_values:2500}

mFd · Accepted Answer · 2021-07-04 19:17:52Z

1

I found a way to get the results that I wanted. Here is the code

db.users.aggregate([
    {
        $project: {
            _id: 1, orders: 1, order_count: { $size: '$orders' }
        }
    },
    { $unwind: '$orders' },
    {
        $project: {
            _id: '$_id', items: '$orders.items', order_count: '$order_count'
        }
    },
    { $unwind: '$items' },
    {
        $project: {
            _id: '$_id', sum: { $sum: '$items.value' }, order_count: '$order_count'
        }
    },
    {
        $group: {
            _id: { _id: '$_id', order_count: '$order_count' }, total_values: { $sum: '$sum' }
        }
    },
])

output:

{ _id: { _id: ObjectId("5dffc33002ef525620ef09f1"), order_count: 2 }, total_values: 1000 }
{ _id: { _id: ObjectId("5dffc33002ef525620ef09f2"), order_count: 3 }, total_values: 1500 }

answered Jul 4, 2021 at 19:17

mFd

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