Getting data of a user that matches the user who created a post

Question

I apologize for the poorly worded title, I wasn't sure how to word it properly. So I have 2 collections:

User

_id:60ccb13a21d65f0c7c4c0690
username: testuser
name: test

And Createpost

_id:60d80b1305dcc535c4bf111a
postTitle: "test post"
postText: "aaaaa"
postUsername: "testuser"

If I wanted to get data of testuser from User displayed on his forum post, what would be the best way to do that? This is what I've tried so far:

router.get('/forum', async (req,res)=>res.render('forum', {newPost: await Createpost.find().sort({ date: 'desc'}), 
postUser: await User.find({"username": Createpost.postUsername})}));

My attempt was to search in the User collection for a matching username field with the postUsername field from the Createpost collection, and then display it in EJS:

<% newPost.forEach(newPost => { %>
    Posted by: <%= newPost.postUsername %> - Name: <%= postUser.name %>
    <%= newPost.postText %>
<% }%>

But when I do this, nothing appears on the page and I can't figure out any solutions. Any help would be much appreciated.

Edit: I used aggregation like suggested in the comments, but the following just results in text that says [object Object] and I'm not sure why:

router.get('/forum', async (req,res)=>res.render('forum', {newPost: await Createpost.find().sort({ date: 'desc'}), 
postUser: Createpost.aggregate([{$lookup: { from: "User", localField: "postUsername", foreignField: "username", as: "postUser"}},{$unset: "postUsername"}])}));

EJS:

<% newPost.forEach(newPost => { %>
    Posted by: <%= newPost.postUsername %> - <%= postUser %>
    <%= newPost.postText %>
<% }%>

Answer 1

Try the aggregate solution: https://docs.mongodb.com/manual/reference/method/db.collection.aggregate/

Createpost.aggregate([
{
    $lookup: {
         from: "User", 
         localField: "postUsername", 
         foreignField: "username", 
         as: "postUser"
    }
},
{    
    $unset: "postUsername"
},
{
    $sort: {date: -1}
}  
])

Edit: The output data from the aggregate should be like this:

[{
    _id: 60d80b1305dcc535c4bf111a,
    postTitle: "test post",
    postText: "aaaaa",
    //postUsername: "testuser", --> removed cuz its dupplicated with the postUser below!
    postUser: {
        _id: 60ccb13a21d65f0c7c4c0690,
        username: "testuser",
        name: "test"
    },
    // ... more detail of the post
}},...
]

More: Now you can render the ejs with just an array newPost

router.get('/forum', async (req,res)=>res.render('forum', {
    newPost: await Createpost.aggregate([
{
    $lookup: {
         from: "User", 
         localField: "postUsername", 
         foreignField: "username", 
         as: "postUser"
    }
},
{    
    $unset: "postUsername"
},
{
    $sort: {data: -1}
}  
]) 
}));

Then parse the newPost array to ejs like below:

<% newPost.forEach(newPost => { %>
    Posted by: <%=newPost.postUser.username%> - Name: <%= newPost.postUser.name%>
    <%= newPost.postText %>
<% }%>