How to group nasted array in mongo db - mongodb

Question

In my mongodb collection I have next records
{ "_id" : ObjectId("5d0dfb68264b2d01a3237a3e"), "name" : "lexa", "cat" : 2, "gender" : "male", "date" : ISODate("2019-06-22T09:56:56.070Z") }
{ "_id" : ObjectId("5d0dfb6c264b2d01a3237a3f"), "name" : "dima", "cat" : 2, "gender" : "male", "date" : ISODate("2019-06-22T09:57:00.925Z") }
{ "_id" : ObjectId("5d0dfb75264b2d01a3237a40"), "name" : "lena", "cat" : 2, "gender" : "female", "date" : ISODate("2019-06-22T09:57:10.003Z") }
{ "_id" : ObjectId("5d0dfb7a264b2d01a3237a41"), "name" : "nina", "cat" : 2, "gender" : "female", "date" : ISODate("2019-06-22T09:57:14.941Z") }
{ "_id" : ObjectId("5d0dfb8f264b2d01a3237a42"), "name" : "nina", "cat" : 1, "gender" : "female", "date" : ISODate("2019-06-22T09:57:35.128Z") }
{ "_id" : ObjectId("5d0dfb93264b2d01a3237a43"), "name" : "lena", "cat" : 1, "gender" : "female", "date" : ISODate("2019-06-22T09:57:39.789Z") }
{ "_id" : ObjectId("5d0dfb9b264b2d01a3237a44"), "name" : "dima", "cat" : 1, "gender" : "male", "date" : ISODate("2019-06-22T09:57:47.150Z")
Then I use aggregation mongo framework to group that records by cat.
db.foo.aggregate([{'$group':
{
'_id': '$cat',
'users':
{'$push':
{
'name':'$name',
'gender': '$gender'
}
}
}
}])
That query returns me next result
{
"_id" : 1,
"users" : [
{
"name" : "nina",
"gender" : "female"
},
{
"name" : "lena",
"gender" : "female"
},
{
"name" : "dima",
"gender" : "male"
}
]
}
{
"_id" : 2,
"users" : [
{
"name" : "lexa",
"gender" : "male"
},
{
"name" : "dima",
"gender" : "male"
},
{
"name" : "lena",
"gender" : "female"
},
{
"name" : "nina",
"gender" : "female"
}
]
}
And the question is what I need to add to my query to group by my users array. I want to get something like this
{
"_id" : 1,
"users" : [
{
"gender": "male",
"names": ["dima"]
},
{
"gender": "female",
"names": ["lena", "nina"]
}
]
}
{
"_id" : 2,
"users" : [
{
"gender": "male",
"names": ["lexa", "dima"]
},
{
"gender": "female",
"names": ["lena", "nina"]
}
]
}
I need to have my nested array been grouped too without loosing first group result

Answer 1

Would be easier to first group by cat X gender and then restructure the data like so :
db.foo.aggregate([
{'$group':
{
'_id': {cat: '$cat', gender: "$gender"},
'names':
{'$push':
{
'name':'$name',
}
}
}
},
{
$group: {
'_id': "$_id.cat",
users: { $push: { gender: "$_id.gender", names: "$names" }
}
}
])