$lookup in MongoDB Provides Unexpected Data Structure Result - mongodb

Question

I am trying to understand why a $lookup I'm using in my MongoDB aggregation is producing the result it is.
First off, my initial data looks like this:
"subscriptions": [
{
"agency": "3dg2672f145d0598be095634", // This is an ObjectId
"memberType": "primary"
}
]
Now, what I want to do is a simple $lookup, pulling in the related data for the ObjectId that's currently being populated as the value to the "agency" field.
What I tried doing was a $lookup like this:
{
"from" : "agencies",
"localField" : "subscriptions.0.agency",
"foreignField" : "_id",
"as" : "subscriptions.0.agency"
}
So, basically what I want to do is go get that info related to that ObjectId ref, and populate it right here, in place of where the ObjectId currently resides.
What I'd expect as a result is something like this:
"subscriptions": [
{
"agency": [
{
_id: <id-value>,
name: <name-value>,
address: <address-value>
}
],
"memberType": "primary"
}
]
Instead, I end up with this (with my "memberType" prop now nowhere to be found):
"subscriptions" : {
"0" : {
"agency" : [ <agency-data> ]
}
}
Why is this the result of the $lookup, and how can I get the data structure I'm looking for here?
To clarify further, in the docs they mention using an $unwind BEFORE the $lookup when it's an array field. But in this case, the actual local field being targeted and replaced by the $lookup is NOT an array, but it is within an array. So I'm not clear on what the problem is.

Answer 1

You need to use $unwind to match your "localField" with to the "foreignField" and then $group to rollback again to the array
db.collection.aggregate([
{ "$unwind": "$subsciption" },
{ "$lookup": {
"from": Agency.collection.name,
"localField": "subsciption.agency",
"foreignField": "_id",
"as": "subsciption.agency"
}},
{ "$group": {
"_id": "$_id",
"memberType": { "$first": "$memberType" },
"subsciption": { "$push": "$subsciption" },
}}
])

Answer 2

Basically, what OP is looking for is to transform data in desired format after looking up into another collection. Assuming there are two collections C1 and C2 where C1 contains document
{ "_id" : ObjectId("5b50b8ebfd2b5637081105c6"), "subscriptions" : [ { "agency" : "3dg", "memberyType" : "primary" } ] }
and C2 contains
{ "_id" : ObjectId("5b50b984fd2b5637081105c8"), "agency" : "3dg", "name" : "ABC", "address" : "1 some street" }
if following query is executed against database
db.C1.aggregate([
{$unwind: "$subscriptions"},
{
$lookup: {
from: "C2",
localField: "subscriptions.agency",
foreignField: "agency",
as: "subscriptions.agency"
}
}
])
We get result
{
"_id": ObjectId("5b50b8ebfd2b5637081105c6"),
"subscriptions": {
"agency": [{
"_id": ObjectId("5b50b984fd2b5637081105c8"),
"agency": "3dg",
"name": "ABC",
"address": "1 some street"
}],
"memberyType": "primary"
}
}
which is pretty close to what OP is looking forward.
Note: there may be some edge cases but with minor tweaks, this solution should work