Referencing root _id in aggregate lookup match expression not working - mongodb

Question

This is my first experience using aggregate pipeline. I'm not able to get a "$match" expression to work inside the pipeline. If I remove the "_id" match, I get every document in the collection past the start date, but once I add the $eq expression, it returns empty.
I read a lot of other examples and tried many different ways, and this seems like it is correct. But the result is empty.
Any suggestions?
let now = new Date()
let doc = await Team.aggregate([
{ $match: { created_by: mongoose.Types.ObjectId(req.params.user_oid)} },
{ $sort: { create_date: 1 } },
{ $lookup: {
from: 'events',
let: { "team_oid": "$team_oid" },
pipeline: [
{ $addFields: { "team_oid" : { "$toObjectId": "$team_oid" }}},
{ $match: {
$expr: {
$and: [
{ $gt: [ "$start", now ] },
{ $eq: [ "$_id", "$$team_oid" ] }
]
},
}
},
{
$sort: { start: 1 }
},
{
$limit: 1
}
],
as: 'events',
}},
{
$group: {
_id: "$_id",
team_name: { $first: "$team_name" },
status: { $first: "$status" },
invited: { $first: "$invited" },
uninvited: { $first: "$uninvited" },
events: { $first: "$events.action" },
dates: { $first: "$events.start" } ,
team_oid: { $first: "$events.team_oid" }
}
}])
Example Docs (added by request)
Events:
_id:ObjectId("60350837c57b3a15a414d265")
invitees:null
accepted:null
sequence:7
team_oid:ObjectId("60350837c57b3a15a414d263")
type:"Calendar Invite"
action:"Huddle"
status:"Questions Issued"
title:"Huddle"
body:"This is a Huddle; you should receive new questions 5 days befor..."
creator_oid:ObjectId("5ff9e50a206b1924dccd691e")
start:2021-02-26T07:00:59.999+00:00
end:2021-02-26T07:30:59.999+00:00
__v:0
Team:
_id:ObjectId("60350837c57b3a15a414d263")
weekly_schedule:1
status:"Live"
huddle_number:2
reminders:2
active:true
created_by:ObjectId("5ff9e50a206b1924dccd691e")
team_name:"tESTI"
create_date:2021-02-23T13:50:47.172+00:00
__v:0

Answer 1

This is just a guess since you don't have schema in your question. But it looks like your have some of your _ids mixed up. Where you are currently trying to $match events whose _id is equal to a team_oid. Rather than the event's team_oid field being equal to the current 'team' _id.
I'm pretty confident this will produce the correct output. If you post any schema or sample docs I will edit it.
https://mongoplayground.net/p/5i1w2Ii7KCR
let now = new Date()
let doc = await Team.aggregate([
{ $match: { created_by: mongoose.Types.ObjectId(req.params.user_oid)} },
{ $sort: { create_date: 1 } },
{ $lookup: {
from: 'events',
// Set tea_oid as the current team _id
let: { "team_oid": "$_id" },
pipeline: [
{ $match: {
$expr: {
$and: [
{ $gt: [ "$start", now ] },
// Match events whose 'team_oid' field matches the 'team' _id set above
{ $eq: [ "$team_oid", "$$team_oid" ] }
]
},
}
},
{
$sort: { start: 1 }
},
{
$limit: 1
}
],
as: 'events',
}},
{
$group: {
_id: "$_id",
team_name: { $first: "$team_name" },
status: { $first: "$status" },
invited: { $first: "$invited" },
uninvited: { $first: "$uninvited" },
events: { $first: "$events.action" },
dates: { $first: "$events.start" } ,
team_oid: { $first: "$events.team_oid" }
}
}])