mongodb aggregation lookup with multiple conditions and ids - mongodb

Question

Having the following collections and data on them
db.a.insert([
{ "_id" : ObjectId("5b56989172ebcb00105e8f41"), "items" : [{id:ObjectId("5b56989172ebcb00105e8f41"), "instock" : 120}]},
{ "_id" : ObjectId("5b56989172ebcb00105e8f42"), "items" : [{id:ObjectId("5b56989172ebcb00105e8f42"), "instock" : 120}] },
{ "_id" : ObjectId("5b56989172ebcb00105e8f43"), "items" : [{ObjectId("5b56989172ebcb00105e8f43"), "instock" : 80}] }
])
db.b.insert([
{ "_id" : ObjectId("5b56989172ebcb00105e8f41")},
{ "_id" : ObjectId("5b56989172ebcb00105e8f42")},
{ "_id" : ObjectId("5b56989172ebcb00105e8f43")},
{ "_id" : ObjectId("5b56989172ebcb00105e8f44")},
{ "_id" : ObjectId("5b56989172ebcb00105e8f45")}
])
executing an lookup aggregation like
db.b.aggregate([
{
$lookup:
{
from: "b",
let: { bId: "$_id", qty: 100 },
pipeline: [
{ $match:
{ $expr:
{ $and:
[
{ $eq: [ "$items.id", "$$bId" ] },
{ $gte: [ "$instock", "$$qty" ] }
]
}
}
}
],
as: "a"
}
}
])
does not bring any results in the expected lookup operation. Is there any restriction to use ObjectId as a comparison? In the official documentations does not say any about it and it works like a charm with any other kind of data type, like strings

Answer 1

I am not sure if this is a bug in mongodb or not but the query only works after adding an $unwind stage first.
db.b.aggregate([
{
$lookup:
{
from: "a",
let: { bId: "$_id", qty: 100 },
pipeline: [
{
$unwind: {
path: "$items"
}
},
{ $match:
{ $expr:
{ $and:
[
{ $eq: [ "$items.id", "$$bId" ] },
{ $gte: [ "$items.instock", "$$qty" ] },
]
}
}
}
],
as: "a"
}
}
]);
Note: Join Conditions and Uncorrelated Sub-queries were added in mongo 3.6