How to write Mongo Aggregation to sort by element inside array

Question

Here is the example:

{_id: 111, sort: [{order:1}, {order:3}]}
{_id: 222, sort: [{order:2}, {order:5}]}
{_id: 333, sort: [{order:1}, {order:3}, {order:4}]}

I want to sort by sort.order from big to small, and get

{_id: 222, sort: {order:5}}
{_id: 333, sort: {order:4}}
{_id: 111, sort: {order:3}}

mickl · Accepted Answer · 2018-01-23 22:06:45Z

2

You need to $unwind them to determine which one has highest value and then group back to take only one:

db.col.aggregate([
    { $unwind: "$sort" },
    { $sort: { "sort.order": -1 } },
    {
        $group: {
            _id: "$_id",
            sort: { $first: "$sort" }
        }
    },
    { $sort: { "sort.order": -1 } },
])

answered Jan 23, 2018 at 22:06

mickl

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