Can't $unwind documents after $group - database

Question

I'm learning Mongo and I'm not understanding something about the aggregate function. I have a User that can make many different kinds of posts, I match the user by blogName and then $lookup all posts that have the matching user ID.
After that I'm stumped. If I unwind and don't group then I get the mess of joined documents, but all I want are the posts.
If I group after $unwind then the documents are turned back into objects.
This code:
return User
.aggregate([
{
$match: {
blogName: blogName
}
},
{
$lookup: {
from: 'posts',
localField: '_id',
foreignField: 'user',
as: 'posts'
}
},
{
$unwind: "$posts"
},
{
$group: {
_id: "$_id",
'posts': { $push: { fields: '$posts' } }
}
},
Gives me this:
[
{
_id: 6065e579bf709d81274cc51e,
posts: [ [Object], [Object], [Object], [Object] ]
}
]
Adding a second $unwind after $group just gives me this:
[
{ _id: 6065e579bf709d81274cc51e, posts: { fields: [Object] } },
{ _id: 6065e579bf709d81274cc51e, posts: { fields: [Object] } },
{ _id: 6065e579bf709d81274cc51e, posts: { fields: [Object] } },
{ _id: 6065e579bf709d81274cc51e, posts: { fields: [Object] } }
]
Which is further from what I want.
I just want the posts. If I can $unwind the posts in this array then I can handle everything from there. What am I not understanding about this?
Update 1:
If I just use $project like so:
return User
.aggregate([
{
$match: {
blogName: blogName
}
},
{
$lookup: {
from: 'posts',
localField: '_id',
foreignField: 'user',
as: 'posts'
}
},
{
$unwind: "$posts"
},
{
$project: {
"_id": 0,
"posts": "$posts"
}
},
I can get this array:
[
{
posts: {
_id: 60664856447970128fee597b,
descriptionImages: [],
tags: [],
likes: [],
kind: 'TextPost',
createdAt: 2021-04-01T22:25:26.531Z,
updatedAt: 2021-04-01T22:25:26.531Z,
title: '',
body: '',
user: 6065e579bf709d81274cc51e,
__v: 0
}
},
{
posts: {
_id: 60664925d2548912dc960e05,
mainImages: [],
descriptionImages: [],
tags: [],
likes: [],
kind: 'PhotoPost',
createdAt: 2021-04-01T22:28:53.179Z,
updatedAt: 2021-04-01T22:28:53.179Z,
user: 6065e579bf709d81274cc51e,
description: '',
__v: 0
}
},
{
posts: {
_id: 6066495f347bb812fd6dc703,
mainImages: [],
descriptionImages: [],
tags: [],
likes: [],
kind: 'PhotoPost',
createdAt: 2021-04-01T22:29:51.815Z,
updatedAt: 2021-04-01T22:29:51.815Z,
user: 6065e579bf709d81274cc51e,
description: '',
__v: 0
}
},
{
posts: {
_id: 60664961347bb812fd6dc704,
descriptionImages: [],
tags: [],
likes: [],
kind: 'TextPost',
createdAt: 2021-04-01T22:29:53.385Z,
updatedAt: 2021-04-01T22:29:53.385Z,
title: '',
body: '',
user: 6065e579bf709d81274cc51e,
__v: 0
}
}
]
But now I'm stumped as to how to get rid of the extra level of nesting. I just need each post object without the additional nesting under posts.

Answer 1

Demo - https://mongoplayground.net/p/bCwBJipyYYZ
db.collection.aggregate([
{ $match: { blogName: "abc" } },
{ $unwind: "$posts" },
{ $replaceRoot: { "newRoot": "$posts" } }
])
Use $replaceRoot
Replaces the input document with the specified document. The operation replaces all existing fields in the input document, including the _id field. You can promote an existing embedded document to the top level, or create a new document for promotion (see example).
User.aggregate([
{ $match: { blogName: blogName } },
{ $lookup: { from: 'posts', localField: '_id', foreignField: 'user', as: 'posts' } },
{ $unwind: "$posts" },
{ $replaceRoot: { "newRoot": "$posts" } }
]