Display mongodb multiple group by query results in a nested format - mongodb

Question

Given the following dataset:
[
{
"account_id" : "1111"
"task_id" : "aaaa",
"workweek" : "20",
"hours": "18"
},
{
"account_id" : "1111"
"task_id" : "aaaa",
"workweek" : "20",
"hours": "12"
},
{
"account_id" : "1111"
"task_id" : "aaaa",
"workweek" : "21",
"hours": "10"
},
{
"account_id" : "1111"
"task_id" : "bbbb",
"workweek" : "21",
"hours": "5"
},
{
"account_id" : "2222"
"task_id" : "cccc",
"workweek" : "21",
"hours": "15"
}
]
I'd like to group the documents and have the results in the following format:
[
{
"account_id": "1111",
"tasks": [
{
"task_id": "aaaa",
"workweeks": [
{
"workweek": "20",
"total_hours": "30"
},
{
"workweek": "21",
"total_hours": "10"
}
]
},
{
"task_id": "bbbb",
"workweeks": [
{
"workweek": "21",
"total_hours": "5"
}
]
}
]
},
{
"account_id": "2222",
"tasks": [
{
"task_id": "cccc",
"workweeks": [
{
"workweek": "21",
"total_hours": "15"
}
]
},
]
}
]
My broken aggregation code is the following:
db.tasks.aggregate([
{"$group": {
"_id": {
"account_id": "$account_id",
"task_id": "$task_id",
"workweek": "$workweek",
},
"total_hours": {$sum: "$hours"}
}},
{"$group": {
"_id": "$_id.account_id",
"tasks": {
"$push": {
"task_id": "$_id.task_id",
"workweeks": {
"$push": {
"workweek": "$_id.workweek",
"total_hours": "$total_hours"
}
}
}
}
}}
]);
I receive this error:
"errmsg" : "exception: invalid operator '$push'",
And I assume it is because I can only $push once per $group block.
Any ideas on how to achieve the desired result?

Answer 1

Please try with one additional (middleware) "group-by" step:
db.tasks.aggregate([
{"$group": {
"_id": {
"account_id": "$account_id",
"task_id": "$task_id",
"workweek": "$workweek",
},
"total_hours": {$sum: "$hours"}
}},
{"$group": {
"_id": {
"$_id.account_id",
"$_id.task_id"
},
"workweeks": {
"$push": {
"workweek": "$_id.workweek",
"total_hours": "$total_hours"
}
}
}},
{"$group": {
"_id": "$_id.account_id",
"tasks": {
"$push": {
"task_id": "$_id.task_id",
"workweeks": "$workweeks"
}
}
}}
]);