Combined lookup and embed in the same MongoDB collection depending on root document structure - mongodb

Question

I want to make a conditional lookup with Mongo in the following way: a root document contains either a link (customerId) to the customers collection or directly embeds a customer, like this:
{
"_id" : 1,
"item": "item1",
"customer": { _id: 1, "name": "Jane Doe" }
},
{
"_id": 2,
"item": "item2",
"customerId": 1
}
customers collection:
{ _id: 1, "name": "Jane Johnson" }
The customers collection stores the current versions of customers; to maintain consistency, members of the items collection will contain just ids of customer. But if I want to freeze an item so it holds the version of its customer at a certain time, I will embed that customer directly into the item in question.
When searching for items I want them to appear uniformly (i.e. regardless whether customer is looked up or embedded it will appear as embedded field):
e.g.
[{
"_id" : 1,
"item": "item1",
"customer": { _id: 1, "name": "Jane Doe" } // historical version of Jane (embedded)
},
{
"_id": 2,
"item": "item2",
"customer": { _id: 1, "name": "Jane Johnson" } // current version of Jane by lookup
}]
Question 1: is this the right approach and if not what is the best practice for handling cases like this?
Question 2: if my approach is correct, how to best use aggregation framework to achieve this?
Thanks!

Answer 1

The answers:
Yes, you can achieve it with aggregation framework and it is one of the possible solutions (the other possible solution would be to implement it in your program).
Just use $lookup (for data merging from other collection) and $project (for final result generation) pipeline stages.
Query example:
db.getCollection('items').aggregate([
{
$lookup: {
from: "customers",
localField: "customerId",
foreignField: "_id",
as: "customerData"
}
},
{
$project: {
"item": 1,
"customer": {
$cond: {
if: {$gt: [{$size: "$customerData"}, 0]},
then: {$arrayElemAt: ["$customerData", 0]},
else: "$customer"
}
}
}
}
]);