Mongoose sort and get only one per field - node.js

Question

I need to get a list of item using the mongoose, however, i need select only one item of any type (i've 5 possible values), and always the last inserted item of this type.
for example, to the list below, not ordered (i know, here it's ordered).
[
{
"_id": "5b226abab64b2309de01bfd5",
"documentType": "type3",
"createdAt": "2018-06-14T13:16:42.321Z"
},
{
"_id": "5b226a5da93b0e09b8aee447",
"documentType": "type3",
"createdAt": "2018-06-14T13:15:09.458Z"
},
{
"_id": "5b226a21f454d6097ffa461c",
"documentType": "type2",
"createdAt": "2018-06-14T13:14:09.159Z"
},
{
"_id": "5b2267fb445d7709590e1695",
"documentType": "type1",
"createdAt": "2018-06-14T13:04:59.742Z"
},
{
"_id": "5b2267de76708b094696b0fe",
"documentType": "type3",
"createdAt": "2018-06-14T13:04:30.349Z"
},
{
"_id": "5b2267ce2f3724092b1a14a3",
"documentType": "type2",
"createdAt": "2018-06-14T13:04:14.410Z"
},
{
"_id": "5b2267a92f3724092b1a14a2",
"documentType": "type4",
"createdAt": "2018-06-14T13:03:37.079Z"
},
{
"_id": "5b22647b63017e08a69a3043",
"documentType": "type5",
"createdAt": "2018-06-14T12:50:03.999Z"
},
{
"_id": "5b2264471778a20880d4ba64",
"documentType": "type5",
"createdAt": "2018-06-14T12:49:11.773Z"
}
]
the expected result is something like this:
[
{
"_id": "5b226abab64b2309de01bfd5",
"documentType": "type3",
"createdAt": "2018-06-14T13:16:42.321Z"
},
{
"_id": "5b226a21f454d6097ffa461c",
"documentType": "type2",
"createdAt": "2018-06-14T13:14:09.159Z"
},
{
"_id": "5b2267fb445d7709590e1695",
"documentType": "type1",
"createdAt": "2018-06-14T13:04:59.742Z"
},
{
"_id": "5b2267a92f3724092b1a14a2",
"documentType": "type4",
"createdAt": "2018-06-14T13:03:37.079Z"
},
{
"_id": "5b22647b63017e08a69a3043",
"documentType": "type5",
"createdAt": "2018-06-14T12:50:03.999Z"
}
]
Basicly only one per type, sort desc by id
How can i do this?

Answer 1

You can use $group by in mongodb to distinct the documentType
db.collection.aggregate([
{ "$group": {
"_id": "$documentType",
"createdAt": { "$first": "$createdAt" },
"id": { "$first": "$_id" }
}},
{ "$project": {
"_id": "$id",
"createdAt": 1,
"documentType": "$_id"
}}
])
The output will be
[
{
"_id": "5b226abab64b2309de01bfd5",
"createdAt": "2018-06-14T13:16:42.321Z",
"documentType": "type3"
},
{
"_id": "5b2267fb445d7709590e1695",
"createdAt": "2018-06-14T13:04:59.742Z",
"documentType": "type1"
},
{
"_id": "5b22647b63017e08a69a3043",
"createdAt": "2018-06-14T12:50:03.999Z",
"documentType": "type5"
},
{
"_id": "5b2267a92f3724092b1a14a2",
"createdAt": "2018-06-14T13:03:37.079Z",
"documentType": "type4"
},
{
"_id": "5b226a21f454d6097ffa461c",
"createdAt": "2018-06-14T13:14:09.159Z",
"documentType": "type2"
}
]