MongoDB Exclude docs from aggregate if field/reference exists in other collection - mongodb

Question

const sellerSchema = Schema(
{
name: String,
url:String
}
const productSchema = Schema(
{
title: String,
sellerUrl:String
}
Below query will return unique sellerUrl from all products:
context.Product.aggregate([
{
$group: {
_id: "$sellerUrl",
}
}
]);
But I also want to exclude from aggregation, sellers that I already saved. So if url == sellerUrl aggregation must exclude that seller.
Please help me

Answer 1

You can try below query :
db.product.aggregate([
{
$group: {
_id: "", /** group on no condition & push all unique `sellerUrl` to sellerUrls array */
sellerUrls: { $addToSet: "$sellerUrl" }
}
},
{
$lookup: {
from: "seller",
let: { sellerUrls: "$sellerUrls" }, // creating local variable
pipeline: [
{ $group: { _id: "", urls: { $addToSet: "$url" } } }, /** group on no condition & push all unique `url` to urls array */
{ $project: { _id: 0, uniqueAndNotInSellerColl: { $setDifference: [ "$$sellerUrls", "$urls" ] } } } // get difference between two arrays
],
as: "data" // As we're grouping will always be one doc/element in an array
}
},
/** Create a new root doc from getting first element(though it will have only one) from `data` array */
{
$replaceRoot: { newRoot: { $arrayElemAt: [ "$data", 0 ] } }
}
])
Test : mongoplayground
Update :
As you need few other fields from product collection but not just the sellerUrl field then try below query :
db.product.aggregate([
{
$group: {
_id: "$sellerUrl",
docs: { $push: { title: "$title" } } // We're only retrieving `title` field from `product` docs, if every field is needed use `$$ROOT`
}
},
/** We've used basic `lookup` stage, use this if you've only few matching docs from `seller` collection
* If you've a lot of matching docs for each `_id` (sellerUrl),
* then instead of getting entire `seller` doc (which is not needed) use `lookup` with aggregation pipeline &
* just get `_id`'s of seller docs for better performace refer previous query
*/
{
$lookup: {
from: "seller",
localField: "_id",
foreignField: "url",
as: "sellerDocs"
}
},
/** match will retain only docs which doesn't have a matching doc in seller collection */
{
$match: { sellerDocs: [] }
},
{
$project: { sellerDocs: 0 }
}
])
Test : mongoplayground