how to show a specific field in array using $project in mongodb - mongodb

Question

I have a database that contains information about flights. I'm trying to find the category that has the least minutes of delays. I managed to find and show the number of the minimum minutes of the category but not the category itself.
I've tried to put ":true" after each field to show it
db.delayData.aggregate([{
$group: {
"_id": "$carrier",
"arr_sum": {
$sum: "$arr_delay"
},
"carrier_sum": {
$sum: "$carrier_delay"
},
"weather_sum": {
$sum: "$weather_delay"
},
"nas_sum": {
$sum: "$nas_delay"
},
"sec_sum": {
$sum: "$security_delay"
},
"late_air_sum": {
$sum: "$late_aircraft_delay"
}
}
},
{
$project {
"min_delay_category": {
$min["$arr_sum", "$carrier_sum", "$weather_sum", "$nas_sum", "$sec_sum", "$late_air_sum"]
}
}
]).pretty()
I want to have something like that:
{ "_id" : "VX", "min_delay_category" : 1449, "sec_sum"... }
I've tried to write:
..."$sec_sum":1,"$late_air_sum":1]
but the error message is:
"missing ] after element list"
when I wrote:
...{"sec_sum":1},{"late_air_sum":1}]
I don't have error message but it will give me the least second result, not the first one.
for example:
{ "_id" : "VX", "min_delay_category" : 69081 }
but the true result for "VX" is 1449

Answer 1

The following query can get us the expected output:
db.collection.aggregate([
{
$project:{
"carrier":1,
"category.arr_delay":"$arr_delay",
"category.carrier_delay":"$carrier_delay",
"category.weather_delay":"$weather_delay",
"category.nas_delay":"$nas_delay",
"category.security_delay":"$security_delay",
"category.late_aircraft_delay":"$late_aircraft_delay"
}
},
{
$project:{
"carrier":1,
"categories":{
$objectToArray:"$category"
}
}
},
{
$unwind:"$categories"
},
{
$group:{
"_id":{
"carrier":"$carrier",
"category":"$categories.k"
},
"carrier":{
$first:"$carrier"
},
"category":{
$first:"$categories.k"
},
"total_delay":{
$sum:"$categories.v"
}
}
},
{
$sort:{
"total_delay":1
}
},
{
$group:{
"_id": "$carrier",
"carrier":{
$first:"$carrier"
},
"category":{
$first:"$category"
},
"minimum_delay":{
$first:"$total_delay"
}
}
},
{
$project:{
"_id":0
}
}
]).pretty();
Data set:
{
"_id" : ObjectId("5d5b5058435c7584459b7bae"),
"year" : 2003,
"month" : 6,
"carrier" : "AA",
"carrier_name" : "American Airlines Inc.",
"airport" : "ABQ",
"airport_name" : "Albuquerque, NM: Albuquerque International Sunport",
"arr_flights" : 307,
"arr_del15" : 56,
"carrier_ct" : 14.68,
"weather_ct" : 10.79,
"nas_ct" : 19.09,
"security_ct" : 1.48,
"late_aircraft_ct" : 9.96,
"arr_cancelled" : 1,
"arr_diverted" : 1,
"arr_delay" : 2530,
"carrier_delay" : 510,
"weather_delay" : 621,
"nas_delay" : 676,
"security_delay" : 25,
"late_aircraft_delay" : 698,
"" : ""
},
{
"_id" : ObjectId("5d5b5058435c7584459b7bbe"),
"year" : 2003,
"month" : 6,
"carrier" : "AA",
"carrier_name" : "American Airlines Inc.",
"airport" : "ABQ",
"airport_name" : "Albuquerque, NM: Albuquerque International Sunport",
"arr_flights" : 307,
"arr_del15" : 56,
"carrier_ct" : 14.68,
"weather_ct" : 10.79,
"nas_ct" : 19.09,
"security_ct" : 1.48,
"late_aircraft_ct" : 9.96,
"arr_cancelled" : 1,
"arr_diverted" : 1,
"arr_delay" : 2530,
"carrier_delay" : 510,
"weather_delay" : 621,
"nas_delay" : 676,
"security_delay" : 2512,
"late_aircraft_delay" : 698,
"" : ""
}
Output:
{ "carrier" : "AA", "category" : "carrier_delay", "minimum_delay" : 1020 }
Aggregation stage details:
STAGE I: Projecting all delays as a part of category document
STAGE II: Converting category into an array of key-value pair
where 'k' is delay type and 'v' is a delay
STAGE III: Unwinding the prepared array
STAGE IV: Grouping on the basis of carrier and delay type(k) and summing up delay for each type
STAGE V: Sorting on total calculated delay in ascending order
STAGE VI: Grouping on carrier and fetching the first document
which holds the minimum delay