Mongodb: sort by elements in an array

Question

I am working on a project and I found a request that I don't understand and is not working. We have documents that look like this:

 {
        "id": ObjectId("6457959757555445"),
        "buyer" : { 
            "id": ObjectId("588888815654684")      
            "roles" : [ 
                "BUYER"
            ]
        }
},
{
        "id": ObjectId("6457959707555445"),
        "buyer" : { 
            "id": ObjectId("588800015654684")      
            "roles" : [ 
                "ADMIN"
            ]
        }
}

And there is a request that should sort a collection of theses documents by roles. So for the example above, I will have ADMIN then BUYER. So here is the request:

{  
   "aggregate":"__collection__",
   "pipeline":[  
      {  
         "$sort":{  
            "buyer.roles":1
         }
    },
}

Should we really use pipeline here? and why it is not working?

Hi.Can you explain better what you want to obtain? roles is an array. How to sort something by an array? — Gianluca Conte
– Gianluca Conte, Commented Jul 10, 2018 at 11:42
Do you need to sort with role name that is "ADMIN" will come first and "BUYER" will second? — Ashh
– Ashh, Commented Jul 10, 2018 at 11:50

Ashh · Accepted Answer · 2018-07-10 11:57:19Z

2

You need to first $unwind "roles" array and then you can easily apply $sort on "roles"

db.collection.aggregate([
  {
    $unwind: "$buyer.roles"
  },
  {
    $sort: {
      "buyer.roles": 1
    }
  }
])

answered Jul 10, 2018 at 11:57

Ashh

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