Project multiple documents with different key nodes for subdocuments - mongodb

Question

I have two documents as shown. Their common factor is a node in the subdocument (type,veg_type). I have also added the same common node to each individual document (udf_type, udf_veg_type).One is a legacy data (with key node veggies) and another is new data (with key node vegetables).
How do I project the combined data of vegetables and veggies(in key node vegs) without the type and veg_type nodes? I use user_id for matching.
Intended Output
{
"user_id": 31,
"veggies": [
{
"udf_type": "green_vegetables",
"tot": 28560,
"itms": [
{
"num": 1,
"itm_det": {
"name": "spinach",
"qty": 18
}
}
],
"chksum": "d1583afab3a04f4b32589cfa64392765n78782ff60a0e0dc24b295868083"
},
{
"udf_type": "vegetables",
"tot": 2860,
"itms": [
{
"num": 1,
"itm_det": {
"name": "onion",
"qty": 1
}
}
],
"chksum": "e497c7b288e50e3be4c6bc676e4c849e4n5645n64a2d77748e185d7a1bce8c"
},
{
"udf_veg_type": "green_vegetables",
"tot": 2352000,
"itms": [
{
"num": 1,
"itm_det": {
"name": "kale",
"qty": 18
}
}
],
"chksum": "87b239cd9b39baa48b4564b5754009a131f542622ba018f37cd1fdb5"
}
]
}
{
"_id" : ObjectId("1"),
"user_id": 31,
"veggies": [
{
"type": "green_vegetables",
"desc": [
{
"udf_type": "green_vegetables",
"tot": 28560,
"itms": [
{
"num": 1,
"itm_det": {
"name": "spinach",
"qty": 18
}
}
],
"chksum": "d1583afab3a04f4b32589cfa64392765n78782ff60a0e0dc24b295868083"
}
]
},
{
"type": "vegetables",
"desc": [
{
"udf_type": "vegetables",
"tot": 2860,
"itms": [
{
"num": 1,
"itm_det": {
"name": "onion",
"qty": 1
}
}
],
"chksum": "e497c7b288e50e3be4c6bc676e4c849e4n5645n64a2d77748e185d7a1bce8c"
}
]
}
]
}
{
"_id" : ObjectId("2"),
"user_id": 31,
"vegetables": [
{
"veg_type": "green_vegetables",
"desc": [
{
"udf_veg_type": "green_vegetables",
"tot": 2352000,
"itms": [
{
"num": 1,
"itm_det": {
"name": "kale",
"qty": 18
}
}
],
"chksum": "87b239cd9b39baa48b4564b5754009a131f542622ba018f37cd1fdb5"
}
]
}
]
}

Answer 1

Assuming that the desc arrays always have just one item, try this:
db.collection.aggregate([
{
$match: {
"user_id": 31 // change this into the user_id variable
}
},
{
$group: {
_id: "$user_id",
veggies: {
$max: "$veggies"
},
vegetables: {
$max: "$vegetables"
}
}
},
{
$project: {
"user_id": "$_id",
"veggies": {
$concatArrays: [
{
$cond: [
"$veggies",
{
$map: {
input: "$veggies",
in: {
$arrayElemAt: [
"$$this.desc",
0
]
}
}
},
[]
]
},
{
$cond: [
"$vegetables",
{
$map: {
input: "$vegetables",
in: {
$arrayElemAt: [
"$$this.desc",
0
]
}
}
},
[]
]
}
]
}
}
}
])