Basta usar
.dot
notação com o name
campo db.foo.aggregate([
{ "$lookup": {
"from": "bar",
"localField": "name",
"foreignField": "foo",
"as": "bars"
}},
{ "$addFields": { "bars": "$bars.name" }}
])
MongoPlayground
.dot
notação com o name
campo db.foo.aggregate([
{ "$lookup": {
"from": "bar",
"localField": "name",
"foreignField": "foo",
"as": "bars"
}},
{ "$addFields": { "bars": "$bars.name" }}
])