how to create an array on the output of a response using aggregate in Mongodb - mongodb

Question

I have in my collection a list of objects with this structure:
[
{
"country": "colombia",
"city":"medellin",
"calification": [
{
"_id": 1,
"stars": 5
},
{
"_id": 2,
"stars": 3
}
]
},
{
"country": "colombia",
"city":"manizales",
"calification": [
{
"_id": 1,
"stars": 5
},
{
"_id": 2,
"stars": 5
}
]
},
{
"country": "argentina",
"city":"buenos aires",
"calification": [
{
"_id": 1,
"stars": 5
},
]
},
{
"country": "perú",
"city":"cusco",
"calification": [
{
"_id": 3,
"stars": 3
},
]
}
]
I am trying to make a filter so that the output is an amount of arrays for each country. this is the example of the output i want.
avg would be result sum 'stars'/ calification.length
{
"colombia": [
{
"city": "medellin",
"avg": 4,
"calification": [
{
"_id": 1,
"stars": 5
},
{
"_id": 2,
"stars": 3
}
]
},
{
"city": "manizales",
"avg": 5,
"calification": [
{
"_id": 1,
"stars": 5
},
{
"_id": 2,
"stars": 3
}
]
}
],
"argentina": {
"city": "buenos aires",
"avg": 5,
"calification": [
{
"_id": 1,
"stars": 5
}
]
},
"peru": {
"city": "cusco",
"avg": 4,
"calification": [
{
"_id": 1,
"stars": 4
}
]
}
}
I am trying to do this:
Alcalde.aggregate([
{
$addFields: {
colombia: {
"$push": {
"$cond": [{ $eq: ["$country", "'Colombia'"] }, true, null]
}
}
}
},
{
$project: { colombia: "$colombia" }
}
]
how can i do it

Answer 1

We can make it more elegant.
MongoDB has $avg operator, let's use it. Also, we can use $group operator to group cities for the same country.
At the end, applying $replaceRoot + $arrayToObject** we transform into desired result.
** it's because we cannot use such expression: {"$country":"$city"}
$replaceRoot $arrayToObject
data : { { [ {
"key" : "val", --> "key" : "val", {k:"key", v: "val"}, --> "key" : "val",
"key2" : "val2" "key2" : "val2" {k:"key2", v: "val2"} "key2" : "val2"
} } ] }
Try this one:
Alcalde.aggregate([
{
$group: {
_id: "$country",
city: {
$push: {
"city": "$city",
"avg": { $avg: "$calification.stars"},
"calification": "$calification"
}
}
}
},
{
$replaceRoot: {
newRoot: {
$arrayToObject: [ [{ "k": "$_id", "v": "$city"}] ]
}
}
}
])
MongoPlayground
EDIT: Generic way to populate city inner object
$$ROOT is variable which stores root document
$mergeObjects adds / override fields to final object
Alcalde.aggregate([
{
$group: {
_id: "$country",
city: {
$push: {
$mergeObjects: [
"$$ROOT",
{
"avg": { "$avg": "$calification.stars" }
}
]
}
}
}
},
{
$project: {
"city.country": 0
}
},
{
$replaceRoot: {
newRoot: {
$arrayToObject: [
[ { "k": "$_id", "v": "$city" } ]
]
}
}
}
])
MongoPlayground