How to match each array field to other field in Mongodb - mongodb

Question

I have an array as below:
const test = [{
"_id": 1,
"name": "apple",
"car": "ford"
},{
"_id": 2,
"name": "melon",
"car": "ferrari"
},{
"_id": 3,
"name": "perl",
"car": "Renaut"
}]
And there is are documents of Mongodb as below:
[{
"name": "perl", "company": "A"
},{
"name": "melon", "company": "B"
},{
"name": "apple", "company": "C"
},{
"name": "apple", "company": "D"
},{
"name": "perl", "company": "E"
},{
"name": "apple", "company": "F"
}]
And I want to get this result using mongodb aggregate:
[{
"name": "perl", "company": "A", testInform: { "_id": 3, "name": "perl", "car": "Renaut"}
},{
"name": "melon", "company": "B", testInform: { "_id": 2, "name": "melon", "car": "ferrari"}
},{
"name": "apple", "company": "C", testInform: { "_id": 1, "name": "apple", "car": "ford"}
},{
"name": "apple", "company": "D", testInform: { "_id": 1, "name": "apple", "car": "ford"}
},{
"name": "perl", "company": "E", testInform: { "_id": 3, "name": "perl", "car": "Renaut"}
},{
"name": "apple", "company": "F", testInform: { "_id": 1, "name": "apple", "car": "ford"}
}]
I think to use aggregate with $match and $facet, etc., but I don't know exactly how to do this. Could you recommend a solution for this?
Thank you so much for reading this.

Answer 1

$lookup with pipeline keyword
db.demo2.aggregate(
{
$lookup:
{
from: "demo1",
let: { recordName: "$name"},
pipeline: [
{ $match:
{ $expr:
{ $and:
[
{ $eq: [ "$$recordName", "$name" ] },
]
}
}
},
],
as: "testInform"
}
}
)

Answer 2

If the test array data is stored in a collection then acheiving O/P is pretty straightforward $lookup with $project aggregation
$arrayElemAt Why? because the lookup would fetch the joined documents in an array as testInform
db.maindocs.aggregate([
{
$lookup: {
from: "testdocs",
localField: "name",
foreignField: "name",
as: "testInform"
}
},
{
$project: {
_id: 0,
name: 1,
company: 1,
testInform: { $arrayElemAt: ["$testInform", 0] }
}
}
]);
Update based on comments:
The idea is to iterate the cursor from the documents stored in mongodb Array.prototype.find() the object from test which matches the name field, add it to result.
const test = [
{
_id: 1,
name: "apple",
car: "ford"
},
{
_id: 2,
name: "melon",
car: "ferrari"
},
{
_id: 3,
name: "perl",
car: "Renaut"
}
];
const cursor = db.collection("maindocs").find();
const result = [];
while (await cursor.hasNext()) {
const doc = await cursor.next();
const found = test.find(e => e.name === doc.name);
if (found) {
doc["testInform"] = found;
}
result.push(doc);
}
console.info("RESULT::", result);

Answer 3

The aggregation has one stage: Iterate over the test array and get the array element as an object which matches the name field in both the document and the array (using the $reduce operator).
const test = [ { ... }, ... ]
db.test_coll.aggregate( [
{
$addFields: {
testInform: {
$reduce: {
input: test,
initialValue: { },
in: {
$cond: [ { $eq: [ "$$this.name", "$name" ] },
{ $mergeObjects: [ "$$this", "$$value" ] },
"$$value"
]
}
}
}
}
}
] )