mongodb aggregation query to include a specific field - mongodb-query

Question

I have a mongodb schema which looks like
{
post_id: {
type: mongoose.Schema.Types.ObjectId,
required: true,
ref: "Post"
},
comment_by: {
type: mongoose.Schema.Types.ObjectId,
required: true,
ref: "User"
},
comment: {
type: String,
required: true
},
parent_comment_id: {
type: mongoose.Schema.Types.ObjectId,
ref: "Comment"
},
is_deleted: {
type: Boolean,
default: false
}
}
I want to group the comments such that
they have common "parent_comment_id" but I also want to include the "comment" field of the
document whose "id" is "parent_comment_id".
This is query I have written
var comments = await Comment.aggregate([
{
"$match": {
post_id: mongoose.Types.ObjectId(post_id)
}
},
{
"$group": {
_id: `$parent_comment_id`,
replies: { "$count": {} }
}
}
])
and the result is
{ _id: new ObjectId("6278e11fa7887263e6e6fada"), replies: 3 },
{ _id: new ObjectId("6278fb9f6a8d30c46eb53a84"), replies: 5 },
{ _id: null, replies: 2 }
Result I want
{ _id: new ObjectId("6278e11fa7887263e6e6fada"), replies: 3 , comment : <comment>},
{ _id: new ObjectId("6278fb9f6a8d30c46eb53a84"), replies: 5 , comment : <comment>},

Answer 1

Welcome heeya joshi!.
You can do something like this:
db.collection.aggregate([
{
$match: {post_id: mongoose.Types.ObjectId(post_id)}
},
{
$addFields: {
parent_comment_id: {$ifNull: ["$parent_comment_id", "$_id"]}
}
},
{
$group: {
_id: "$parent_comment_id",
"comment": {
$push: {
$cond: [
{$eq: ["$parent_comment_id", "$_id"]},
"$comment",
"$$REMOVE"
]
}
},
replies: {"$count": {}}
}
},
{
$project: {
comment: {$arrayElemAt: ["$comment", 0]},
replies: 1
}
}
])
As you can see here.
After your $match, the addFields adds the parent_comment_id to the parents themselves. Then the $group keeps the comment only for the parents.