如何在mongoose中使用聚合填充嵌套在对象数组中的字段?

Question

我试图在一个数组中填充一个深度嵌套的对象，但没有运气。我尝试展开数组，但生成的数组被转换为对象而不是数组。

let aggQuery: any = [
{ $match: {} }

{
$lookup: {
from: "tribe-answers", localField: "answers", foreignField: "_id", as: "answers"
}
},
{
"$unwind": {
"path": "$answers.createdBy",
"preserveNullAndEmptyArrays": true
}
},
{
$lookup: {
from: "users", localField: "answers.createdBy", foreignField: "_id", as: "answers.createdBy"
}
},

];

预期输出

answers = [
{ _id,
content
createdBy{
_id
firstname
lastname
}
},
{ _id,
content
createdBy{
_id
firstname
lastname
}
}
]

返回输出

answers = {
_id,
content
createdBy{
_id
firstname
lastname
}
}

上面的查询确实填充了createdBy字段，但将数组转换为对象。我需要的是在答案数组的每个元素中填充createdBy字段。谢谢你的帮助。

Answer 1

db.getCollection('tribe-posts').aggregate([
{ 
"$lookup": {
"from": "tribe-answers",
"let": {answerIds: "$answers"},
"pipeline": [
{ "$match": { "$expr": { "$in": [ "$_id", "$$answerIds" ] } } },
{$lookup: { from: "users", localField: "createdBy", foreignField: "_id", as: "createdBy" }},
{"$unwind": {"path": "$createdBy","preserveNullAndEmptyArrays": true} }

],
"as": "answers"
}    
},
])

这将产生预期的输出

answers = [
{ _id,
content
createdBy{
_id
firstname
lastname
}
},
{ _id,
content
createdBy{
_id
firstname
lastname
}
}
]