mongodb $lookup has_many association from embedded document - json

Question

I have a boards collection, a lists collection, and a cards collection. An array of lists is embedded in a board document. I am trying to get an output that looks like this:
{
_id: 1,
title: "a board",
lists: [
{
_id: 1,
title: "a list",
cards: [ { _id: 1, title: "a card", list_id: 1 }, { _id: 2, title: "another card", list_id: 1 } ]
},
...
]
}
I want to nest the cards in the list it belongs to. The card document has a list_id field. I tried this:
db.boards.aggregate([
{ '$match' => { _id: 1 } },
{ '$lookup' => {
from: "cards",
localField: "lists._id",
foreignField: "list_id",
as: "cards"
} },
])
But that resulted in:
{
_id: 1,
title: "a board",
lists: [ { _id: 1, title: "a list" } ],
cards: [ { _id: 1, title: "a card", list_id: 1 }, { _id: 2, title: "another card", list_id: 1 } ]
}
I know that I have to use $unwind to get the result I want, but I can't get it to work

Answer 1

You need one additional aggregation pipeline step to "merge" these two lists and you can achieve it by running $map with embedded $filter. It simply works as a "join" operation on two arrays:
{
$project: {
_id: 1,
title: 1,
lists: {
$map: {
input: "$lists",
as: "list",
in: {
$mergeObjects: [
"$$list",
{
cards: {
$filter: {
input: "$cards",
cond: { $eq: [ "$$this.list_id", "$$list._id" ] }
}
}
}
]
}
}
}
}
}
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