In MongoDB / mongoose sort and limit result of $push aggregate

Question

I want to order and limit the results of and agregate, $group and $push.

Let's say that I have a system with online orders. And I want to get a list with 3 most ordered products each year sorted by year and products amount.

year    product     amount

2016    product A   100
2016    product B   200
2016    product C   300
2016    product D   1000
2016    product E   50

2015    product F   100
2015    product G   800
2015    product A   300
2015    product B   400
2015    product C   100

2014    product A   70
2014    product B   50
2014    product G   500
2014    product C   600
2014    product D   900

The returned object should be something like this:

[
    {
        year: 2016,
        products: [
            {
                product: 'product D',
                amount: 1000
            },
            {
                product: 'product C',
                amount: 300
            },
            {
                product: 'product B',
                amount: 200
            }
        ]
    },
    {
        year: 2015,
        products: [
            {
                product: 'product G',
                amount: 800
            },
            {
                product: 'product B',
                amount: 400
            },
            {
                product: 'product A',
                amount: 300
            }
        ]
    },
    ...
]

In mongoose I can group data by year:

Orders.model
    .aggregate([
        {
            $group: {
                _id: {
                    'year': '$year',
                },
                products: {
                    $push: {
                        product: '$product',
                        amount: '$amount'
                    }
                }
            }
        },

        {
            $project: {
                _id: 0,
                year: '$_id.year',
                products: '$products'
            }
        }
    ])

It will give me all product names and amounts and not sorted.

By I don't know how to sort and limit the grouped data. I tried $sort, $limit, $each without success.

Answer 1

Sort your data before grouping. Then at the end you can use $slice to get the top three elements:

[
        {
            $sort: {
                year:-1,
                amount:-1
            }
        },
        {
            $group: {
                _id: {
                    'year': '$year',
                },
                products: {
                    $push: {
                        product: '$product',
                        amount: '$amount'
                    }
                }
            }
        },
        {
            $project: {
                _id: 0,
                year: '$_id.year',
                product: {$slice: [ "$products", 3 ]}
            }
        }
]