mongodb - using join on a local variable - arrays

Question

I'm using node.js and mongodb, I have an array of objects which holds the names of an id. Let's say below is my array
let names = [
{ value: 1, text: 'One' },
{ value: 2, text: 'Two' },
{ value: 3, text: 'Three' },
{ value: 4, text: 'Gour' }
]
And this is my query result of a collection using $group which gives me the distinct values.
[
{ _id: { code: '1', number: 5 } },
{ _id: { code: '2', number: 5 } },
{ _id: { code: '3', number: 2 } },
{ _id: { code: '4', number: 22 } },
]
$lookup let's us to join the data from a different collection, but in my case I have an array which holds the text value for each of the codes which I got from the query.
Is there a way we can map the text from the array to the results from mongodb?
Any help will be much appreciated.
EDIT
MongoDB query which I was trying
db.collection.aggregate([
{
$match: {
_Id: id
}
},
{
$lookup: {
localField: "code",
from: names,
foreignField: "value",
as: "renderedNames"
}
},
{
"$group" : {
"_id": {
code: "$code",
number: "$number"
}
}
}
]);

Answer 1

Local variable lives in nodejs app, and mongodb knows nothing about it.
It looks like it belongs to representation layer, where you want to show codes as meaningful names. The mapping should be done there. I believe find is the most suitable here:
names.find(name => name.code === doc._id.code).text
If the names are not truly variable but quite constant, you can move it to own collection, e.g. codeNames:
db.codeNames.insert([
{ _id: "1", text: 'One' },
{ _id: "2", text: 'Two' },
{ _id: "3", text: 'Three' },
{ _id: "4", text: 'Gour' }
]);
and use $lookup as following:
db.collection.aggregate([
{
$match: {
_Id: id
}
},
{
"$group" : {
"_id": {
code: "$code",
number: "$number"
}
}
},
{
$lookup: {
localField: "_id.code",
from: "codeNames",
foreignField: "_id",
as: "renderedNames"
}
}
]);
If none of the above suit your usecase, you can pass the names to the database in each request to map names db-side, but you must be really really sure you cannot use 2 previous options:
db.collection.aggregate([
{
$match: {
_Id: id
}
},
{
"$group" : {
"_id": {
code: "$code",
number: "$number"
}
}
},
{
$project: {
renderedNames: { $filter: {
input: [
{ value: "1", text: 'One' },
{ value: "2", text: 'Two' },
{ value: "3", text: 'Three' },
{ value: "4", text: 'Gour' }
],
as: "name",
cond: { $eq: [ "$$name.value", "$_id.code" ] }
}
}
}
},
]);
As a side note, I find $match: {_Id: id} quite confusing, especially followed by $group. If _Id is _id, it is unique. You can have no more than 1 document after this stage, so there is not too much to group really.