如何使用mongodb聚合在嵌套数组中转换文档

Question

My document looks like as shown below and I want to transform it using aggregation. Inside favourite_products I have product array and it has product_id corresponding to each shops. Now I just want product ids of all shops but sorted based on time. :

"favourite_products": [
                {
                    "shop": {
                        "shop_id": "59465888f7babb000485574b",
                        "time": "2017-07-12T06:11:19.817Z"
                    },
                    "product": [
                        {
                            "product_id": "594c2d56f7afcf00043b1195",
                            "time": "2017-07-12T06:10:36.775Z"
                        },
                        {
                            "product_id": "594ac36c76de720004e819f6",
                            "time": "2017-07-12T06:11:19.817Z"
                        }
                    ]
                },
                {
                    "shop": {
                        "shop_id": "593acc24a2902d0004211f1f",
                        "time": "2017-07-12T06:12:59.372Z"
                    },
                    "product": [
                        {
                            "product_id": "594ac36c76de720004e819f6",
                            "time": "2017-07-12T06:12:59.372Z"
                        }
                    ]
                }
            ]

I want to transform it into this:

"favourite_products"

 ["59465888f7babb000485574b",594c2d56f7afcf00043b1195","594ac36c76de720004e819f6","593acc24a2902d0004211f1f","594ac36c76de720004e819f6"]

Answer 1

Below returns time ordered documents of favourite_products.product.product_id

use project if you want the result as different documents. or use group if you want the result as the array in one document.

db['testing-aggregate'].aggregate([
    {$unwind:'$favourite_products'},
    {$unwind:'$favourite_products.product'},
    {$sort:{'favourite_products.product.time':1}}, // order by time. 1=ascending | -1=descending 
    //  {$project:{
    //      '_id':0, // exclude _id from output
    //      'favourite_products':'$favourite_products.product.product_id' // return only product_id
    //  }},
    {$group:{
        _id:null,
        product_id:{$push:'$favourite_products.product.product_id'}
    }}  
])