Mongodb $lookup with nested document - mongodb

Question

I am trying to create a join using mongo's lookup. I have these three collections.
orderTracking
{
_id: ObejctId("59fb7815b3b8429f4750b0df"),
itemName : "Hamam Soap",
TrackLocation: [{locationId: 1, at:"2017-10-11"},
{locationId: 2,at:"2017-10-13"}],
userId : 12,
price: 20
}
locationType
{
_id: ObejctId("59b2111345cb72345a35fefd"),
locationId : 1
productTypeName: "Warehouse"
},{
_id: ObejctId("59af8ce445cb72345a35feea"),
locationId : 2
productTypeName: "On Transit"
}
User
{
_id: ObejctId("59a504eb6171b554c02292a9"),
"user ID":12,
"userName" : "Shahabaz Shafi",
"dateOfBirth" : "1992-01-01",
"addres": {
"country" : "India",
"state" : "Karnataka",
"city" : "Bengaluru"
}
}
and trying to flatten this to this kind of output.
{
"userName" : "Shahabaz Shafi",
"userId":12,
"dateOfBirth" : "1992-01-01",
"country" : "India",
"state" : "Karnataka",
"city" : "Bengaluru"
"locationType" : [ {productTypeName: "Warehouse",at:"2017-10-11"}, {productTypeName: "On Transit",at:"2017-10-13"}]
}
Edit: 15-11-2018 Updated output
Made some changes to the output columns
{
"userName":"Shahabaz Shafi",
"userId":12,
"dateOfBirth":"1992-01-01",
"country":"India",
"state":"Karnataka",
"city":"Bengaluru",
"items":[
{
"itemName":"Hamam Soap",
"userId":12,
"price":20,
"TrackLocation":[
{
"locationId":1,
"at":"2017-10-11",
"productTypeName":"Warehouse"
},
{
"locationId":2,
"at":"2017-10-13",
"productTypeName":"On Transit"
}
]
}
]
}
How do I approach this ?
PS : I am also using compass

Answer 1

You can use below aggregation with mongodb 3.6 and above
db.User.aggregate([
{ "$lookup": {
"from": "orderTracking",
"let": { "userId": "$userId" },
"pipeline": [
{ "$match": { "$expr": { "$eq": ["$userId", "$$userId"] }}},
{ "$unwind": "$TrackLocation" },
{ "$lookup": {
"from": "locationType",
"let": { "location": "$TrackLocation.locationId" },
"pipeline": [
{ "$match": { "$expr": { "$eq": ["$locationId", "$$location"] }}}
],
"as": "locationType"
}},
{ "$project": {
"_id": 0,
"productTypeName": { "$arrayElemAt": ["$locationType.productTypeName", 0] },
"at": "$TrackLocation.at"
}}
],
"as": "locationType"
}},
{ "$replaceRoot": { "newRoot": { "$mergeObjects": ["$addres", "$$ROOT"] }}},
{ "$project": { "addres": 0 }}
])
Output
[
{
"_id": ObjectId("59a504eb6171b554c02292a9"),
"city": "Bengaluru",
"country": "India",
"dateOfBirth": "1992-01-01",
"locationType": [
{
"at": "2017-10-11",
"productTypeName": "Warehouse"
},
{
"at": "2017-10-13",
"productTypeName": "On Transit"
}
],
"state": "Karnataka",
"userId": 12,
"userName": "Shahabaz Shafi"
}
]