Mongoose sort and get non repeated values - arrays

Question

im new in all NodeJS/MongoDB/Mongoose tecnologies, and im trying to get non repeated values from an array, and im sorting the data by two values; location and created_at.
This is the code
Temperature.find().sort({'location': -1,'created_at': -1}).exec(function(err, results){
res.json(results);
});
How can i get the first non repeated value from the array?
This is the array:
[ {
temperature: '24',
humidity: '15',
location: 'Camara4',
created_at: 2017-01-24T21:40:21.552Z
},
{
temperature: '23',
humidity: '15',
location: 'Camara4',
created_at: 2017-01-24T01:18:26.328Z
},
{
temperature: '15',
humidity: '12',
location: 'Camara3',
created_at: 2017-01-26T18:53:34.447Z,
},
{
temperature: '36',
humidity: '11',
location: 'Camara3',
created_at: 2017-01-24T21:41:07.094Z,
},
{
temperature: '35',
humidity: '11',
location: 'Camara3',
created_at: 2017-01-24T21:41:03.092Z,
}
]
and i expect:
[ {
temperature: '24',
humidity: '15',
location: 'Camara4',
created_at: 2017-01-24T21:40:21.552Z
},
{
temperature: '15',
humidity: '12',
location: 'Camara3',
created_at: 2017-01-26T18:53:34.447Z,
},
]

Answer 1

You'll need to use aggregation framework.
$group on location key with $first operator to pick the first value from the sorted order in each group.
Temperature.aggregate({
"$sort": {
'location': -1,
'created_at': -1
}
}, {
$group: {
"_id": "$location",
"result": {
"$first": "$$ROOT"
}
}
}).exec(function(err, results) {
res.json(results);
});
Output:
{
"_id" : "Camara4",
"result" : {
"temperature" : "24",
"humidity" : "15",
"location" : "Camara4",
"created_at" : ISODate("2017-01-24T21:40:21.552Z")
}
}
{
"_id" : "Camara3",
"result" : {
"temperature" : "15",
"humidity" : "12",
"location" : "Camara3",
"created_at" : ISODate("2017-01-26T18:53:34.447Z")
}
}