How to group by multiple keys and values - mongodb

Question

I have a collection of documents where each document has a nestes field outside with two values:
_id: 9287645ztiu234jgk2j3g5jh,
outside: {
temperature: 'low', // 'low' or 'high'
humidity: 'high', // 'low' or 'high'
},
... some more fields
temperature and humidity can have value low or high
I want to count how many times temperature: low, temperature: high, humidity: low, humidity: high is present in each document of the collection, so the query result for e.g. 14 documents should look like this:
{
temperatureLow: 2,
temperatureHigh: 12,
humidityLow: 8,
humidityHigh: 6,
}
I tried a $group (as the only stage in the aggregation pipeline) like this:
$group: {
_id: { temperature: '$outside.temperature', humidity: '$outside.humidity' },
count: { $sum: 1 },
},
And this gives me these documents (EDITED, first post had wrong data):
{
"_id": {
"temperature": "high",
"humidity": "high"
},
"count": 6
},
{
"_id": {
"temperature": "high",
"humidity": "low"
},
"count": 6
},
{
"_id": {
"temperature": "low",
"humidity": "low"
},
"count": 2
}
How can it be combined into on document?

Answer 1

It's possible. You need add project stage with the using cont operator before group:
{
$project: {
"temperatureLow": { $cond: { if: { $eq: ["$outside.temperature", "low"] }, then: 1, else: 0 }},
"temperatureHigh": { $cond: { if: { $eq: ["$outside.temperature", "high"] }, then: 1, else: 0 }},
"humidityLow": { $cond: { if: { $eq: ["$outside.humidity", "low"] }, then: 1, else: 0 }},
"humidityHigh": { $cond: { if: { $eq: ["$outside.humidity", "high"] }, then: 1, else: 0 }}
}
},
{
$group: {
_id: "result",
"temperatureLow": {$sum: "$temperatureLow"},
"temperatureHigh": {$sum: "$temperatureHigh"},
"humidityLow": {$sum: "$humidityLow"},
"humidityHigh": {$sum: "$humidityHigh"},
}
},
Update
or as notes Neil Lunn I can use cond inside sum operator without project stage:
{
$group: {
_id: "result",
"temperatureLow": {$sum: { $cond: { if: { $eq: ["$outside.temperature", "low"] }, then: 1, else: 0 }}},
"temperatureHigh": {$sum: { $cond: { if: { $eq: ["$outside.temperature", "high"] }, then: 1, else: 0 }}},
"humidityLow": {$sum:{ $cond: { if: { $eq: ["$outside.humidity", "low"] }, then: 1, else: 0 }}},
"humidityHigh": {$sum:{ $cond: { if: { $eq: ["$outside.humidity", "high"] }, then: 1, else: 0 }}}
}
},