How to $lookup/populate an embedded document that is inside an array? - mongodb

Question

How to $lookup/populate an embedded document that is inside an array?
Below is how my schema is looking like.
const CommentSchema = new mongoose.Schema({
commentText:{
type:String,
required: true
},
arrayOfReplies: [{
replyText:{
type:String,
required: true
},
replier: [{
type: mongoose.Schema.Types.ObjectId,
ref: 'User',
required: true,
}],
}],
});
How can I get query results that look like below:
[
{
commentText: 'comment text',
arrayOfReplies: [
{
replyText: 'replyText',
replier: {
username:"username"
bio: 'bio'
}
}
]
}
]
I am trying to populate the replier field inside the array arrayOfReplies. I have tried several variations of the aggregation query below. The ones that have come close to what I am trying to achieve have one short-coming. The comments that do not have replies have an arrayOfReplies array that has an empty object. I.e arrayOfReplies: [{}], essentially meaning that the array is not empty.
I have tried using add fields, $mergeObjects among other pipeline operators but to no avail.
How to $lookup/populate the replier document that is inside the arrayOfReplies array?
Below is a template of the main part of my aggregation query, minus trying populate the replier document.
Comment.aggregate([
{$unwind: {"path": '$arrayOfReplies', "preserveNullAndEmptyArrays": true }},
{$lookup:{from:"users",localField:"$arrayOfReplies.replier",foreignField:"_id",as:"replier"}},
{$unwind: {"path": "$replier", "preserveNullAndEmptyArrays": true }},
{$group: {
_id : '$_id',
commentText:{$first: '$commentText'},
userWhoPostedThisComment:{$first: '$userWhoPostedThisComment'},
arrayOfReplies: {$push: '$arrayOfReplies' },
}},

Answer 1

After your lookup stage, each document will have
{
commentText: "text",
arrayOfReplies: <single reply, with replier ID>
replier: [<looked up replier data>]
}
Use an $addFields stage to move that replier data inside the reply object before the group, like:
{$addFields: {"arrayOfReplies.replier":"$replier"}}
Then your group stage will rebuild arrayOfReplies like you want.

Answer 2

You can use the following aggregate:
Playground
Comment.aggregate([
{
$unwind: {
"path": "$arrayOfReplies",
"preserveNullAndEmptyArrays": true
}
},
{
$lookup: {
from: "users",
localField: "arrayOfReplies.replier",
foreignField: "_id",
as: "replier"
}
},
{
$addFields: {
"arrayOfReplies.replier": {
$arrayElemAt: [
"$replier",
0
]
}
}
},
{
$project: {
"replier": 0
}
},
{
$group: {
_id: "$_id",
"arrayOfReplies": {
"$push": "$arrayOfReplies"
},
commentText: {
"$first": "$commentText"
}
}
}
]);

Answer 3

All the answers provided did not solve this issue as stated in the question.
I am trying to populate the replier field inside the array
arrayOfReplies. I have tried several variations of the aggregation
query below. The ones that have come close to what I am trying to
achieve have one short-coming. The comments that do not have replies
have an arrayOfReplies array that has an empty object. I.e
arrayOfReplies: [{}], essentially meaning that the array is not empty.
I wanted an aggregation that returns an empty array (not an array with an empty object) when the array is empty.
I was able to achieve what I wanted by using the code below:
arrayOfReplies:
{$cond:{
if: { $eq: ['$arrayOfReplies', {} ] },
then: "$$REMOVE",
else: {
_id : '$arrayOfReplies._id',
replyText:'$arrayOfReplies.replyText',
}
}}
If you combine the code above with #SuleymanSah's answer you get the full working code.