如何在mongodb中按多个字段进行聚合

Question

I want to aggregate by 2 fields and wish to have the gouping done in a nested way. How do I achieve it? Currently I do the grouping in following way

var query = [
  { '$group': {
    '_id': {
      'employee': '$employee',
      'sector': '$sector'
    },
    'bookCount': { '$sum': 1 }
  }},
  { '$sort': { 'count': -1 } }
];

Order.aggregate(query, function(err, results){
  res.json(results)
});

I want the results to be in the form

{abc:{sector1:4, sector3:5}, xyz: {sector1:10, sector2:23}}

where abc, xyz are employees and sector1, sector2 are sectors.

How do I aggregate to get nested results?

My original document is

[
  {
    "sector": "sector1",
    "employee": "xyz"
  },
  {
    "sector": "sector1",
    "employee": "abc"
  },
  {
    "sector": "sector1",
    "employee": "abc"
  },
  {
    "sector": "sector2",
    "employee": "abc"
  }
]

I want result to be of the form

{abc:{sector1:2,sector2:2}, xyz: {sector1:1}}

Answer 1

You cannot use "data" as "key names" in the aggregation framework, and nor can you create nested objects with nested properties. You shouldn't want to either, as this is an "anti-pattern". Data is data and should stay that way. Plus, there are better ways to do this:

Order.aggregate([
    { "$group": {
        "_id": {
           "employee": "$employee",
           "sector": "$sector"
        },
        "count": { "$sum": 1 }
    }},
    { "$group": {
       "_id": "$_id.employee",
       "sectors": { 
           "$push": {
               "sector": "$_id.sector",
               "count": "$count"
           }
       }
    }}
],function(err,docs) {

});

Which returns a structure like this:

[
    {
            "_id" : "xyz",
            "sectors" : [
                    {
                            "sector" : "sector1",
                            "count" : 1
                    }
            ]
    },
    {
            "_id" : "abc",
            "sectors" : [
                    {
                            "sector" : "sector2",
                            "count" : 1
                    },
                    {
                            "sector" : "sector1",
                            "count" : 2
                    }
            ]
    }
]

So you have a primary grouping key for "employee" values and the other results are "pushed" to an array.

It's a much better structure with consistent results in the naming of keys.