Writing an aggregation query in MongodB - mongodb

Question

Here is my sample document structure e.g.
{
"my_object": {
"1": {
"seq": "1",
"time": "xyz",
},
"2": {
"seq": "2",
"time": "abc",
"sub_aray": {
"0": {
"value": 10
},
"1": {
"value": 10
},
"2": {
"value": -10
}
}
}
}
}
So what I want to achieve is, Sum of all the sub_array's value if exists, if sub_array isn't found, default it to 0
{"seq" : "1", "sub_array" : 0},
{"seq" : "2", "sub_array" : 10}
My Mongo version is 3.4.6 and I am using PyMongo as my driver.

Answer 1

You can do as below
playground
db.collection.aggregate([
{
$project: {
"array": { //To remove dynamic keys - 1,2,etc
"$objectToArray": "$my_object"
}
}
},
{//reshaping array
"$unwind": "$array"
},
{
$project: {//reshaping sub array to access via a static name
"k": {
"$objectToArray": "$array.v.sub_aray"
},
"seq": "$array.v.seq"
}
},
{
"$project": {//actual logic, output structure
"sub_array": {
$sum: "$k.v.value"
},
"_id": 0,
"seq": 1
}
}
])

Answer 2

The trick part is to use the $objectToArray operator to iterate my_object items.
db.collection.aggregate([
{
"$project": {
"my_object": {
"$map": {
input: {
"$objectToArray": "$my_object"
},
as: "obj",
in: {
seq: "$$obj.v.seq",
sub_array: {
$sum: {
$map: {
input: {
"$objectToArray": "$$obj.v.sub_aray"
},
as: "sub",
in: "$$sub.v.value"
}
}
}
}
}
}
}
},
{
"$unwind": "$my_object"
},
{
"$replaceRoot": {
"newRoot": "$my_object"
}
}
])
MongoPlayground