Get collection with changed one field - node.js

Question

I have a problem with getting a collection array with changed one field in each document.
My document is:
{postId: "123123", author: "someId"}
And I want to search by author id in other collection and replace author with author name instead of id
Is there any simple way to do that?
EDIT:
I've wrote something like this
{
$lookup: {
as: "USER",
foreignField: "created",
from: "Users",
localField: "userName"
}
}
Then i recive array of documents from Users collections. I wan't only one based on the value from the field of collection posts
EDIT:
Sample documents from collections:
POSTS:
{
_id: 'p7c3d',
author: 'p9jhkl',
content: "Hello",
createdAt: '12-12-2019',
tags: ['hello'];
}
USERS:
{
_id: 'p9jhkl',
name: 'John Smith'
}
And I wan't that "John Smith" to be in post author field instead of user id.
Edit:
Picture of actual documents

Answer 1

Assuming there is only 1-to-1 relation i.e; documents in POSTS can have only one matching document in USERS.
POSTS :
/* 1 */
{
"_id" : "p7c3d",
"author" : "p9jhkl",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 2 */
{
"_id" : "p7c3d11",
"author" : "p9jhkl11",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 3 */
{
"_id" : "p7c3d1122",
"author" : "p9jhkl1122",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 4 */
{
"_id" : "p7c3d No",
"author" : "p9jhkl No",
"content" : "Hello",
"tags" : [
"hello"
]
}
USERS :
/* 1 */
{
"_id" : "p9jhkl",
"name" : "John Smith"
}
/* 2 */
{
"_id" : "p9jhkl11",
"name" : "John Smith11"
}
/* 3 */
{
"_id" : "p9jhkl1122",
"name" : "John Smith1122"
}
Please try this :
db.POSTS.aggregate([{
$lookup:
{
from: "USERS",
localField: "author",
foreignField: "_id",
as: "userDetails"
}
},{$addFields : {author: { $arrayElemAt: [ '$userDetails.name', 0 ] }}},
{$project : { userDetails: 0}}])
Result :
/* 1 */
{
"_id" : "p7c3d",
"author" : "John Smith",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 2 */
{
"_id" : "p7c3d11",
"author" : "John Smith11",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 3 */
{
"_id" : "p7c3d1122",
"author" : "John Smith1122",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 4 */
{
"_id" : "p7c3d No",
"content" : "Hello",
"tags" : [
"hello"
]
}
Above query would completely remove the field author from documents of POSTS collection where there is no match with USERS documents (this would help to quickly remove POSTS docs from final o/p which has no match with USERS using {$match : {author: {$exists : true}}}), Just in case you need all documents from POSTS as is irrespective of whether there is a match or not, Check document 4 in results for clarification, Please try this :
db.POSTS.aggregate([{
$lookup:
{
from: "USERS",
localField: "author",
foreignField: "_id",
as: "userDetails"
}
},{$addFields :
{author: {
$cond: { if: { $ne: [ "$userDetails", [] ] },
then : { $arrayElemAt: [ '$userDetails.name', 0 ] },
else : '$author'}}}}, {$project : { userDetails: 0}}])
Result :
/* 1 */
{
"_id" : "p7c3d",
"author" : "John Smith",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 2 */
{
"_id" : "p7c3d11",
"author" : "John Smith11",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 3 */
{
"_id" : "p7c3d1122",
"author" : "John Smith1122",
"content" : "Hello",
"tags" : [
"hello"
]
}
/* 4 */
{
"_id" : "p7c3d No",
"author" : "p9jhkl No",
"content" : "Hello",
"tags" : [
"hello"
]
}
With actual data :
db.POSTS.aggregate([{ $addFields: { "createdBy": { "$toObjectId": "$createdBy" } } }, {
$lookup:
{
from: "USERS",
localField: "createdBy",
foreignField: "_id",
as: "userDetails"
}
}, {
$addFields:
{
createdBy: {
$cond: {
if: { $ne: ["$userDetails", []] },
then: { $arrayElemAt: ['$userDetails.name', 0] },
else: { "$toString": "$createdBy" }
}
}
}
}, { $project: { userDetails: 0 } }])

Answer 2

This can be done in three stages:
Match the author in posts to only get posts for an author
Join to authors using the created field
Project the fields you want, and only include the author's name in the result.
Example Below:
db.getCollection("posts").aggregate(
[
{
$match: {
created: ObjectId("5dd446cf3ccb4305e7fbec96")
}
},
{
$lookup: {from: "authors",
let: { id: "$created" },
pipeline: [
{
$match: {
$expr:
{
$eq: ["$$id","$_id"] }
}
},
{ $project: { _id: 0, name: 1 } },
],
as: "author"
}
},
{
$project: {
author: {
$arrayElemAt: ["$author.name", 0]
}
}
},
]
);