Nested grouping with MongoDB

Question

Given a database the form of

[
{ gender: \"m\", age: 1, name: \"A\" },
{ gender: \"f\", age: 2, name: \"B\" },
{ gender: \"m\", age: 3, name: \"C\" },
{ gender:          


        

Answer 1

Something like this should do it;

db.example.aggregate( 
  { 
    $group: { 
      _id:   { age: "$age", gender: "$gender" }, 
      names: { $addToSet: "$name" } 
    } 
  }, 
  { 
    $group: {
      _id: { age: "$_id.age" }, 
      children: { $addToSet: { gender: "$_id.gender", names:"$names" } } 
    } 
  } 
)

...which gives the result;

{
  "_id" : {
    "age" : 1
  },
  "children" : [
    { "gender" : "m", "names" : [ "G", "A" ] },
    { "gender" : "f", "names" : [ "J", "D" ] }
  ]
}, 
...

If you want the age as _id as in your example, just replace the second grouping's _id by;

_id: "$_id.age",