nodejs/mongodb - Aggregate items by substring - node.js

Question

I have two related collections that contain documents as follows:
/* heroes */
{ id: "HID_1", name: "A" }
{ id: "HID_2", name: "B" }
/* weapons */
{ name: "WHID_1", weapon: "Sword" }
{ name: "WHID_2", weapon: "Lance" }
How can I aggregate them so I get a single document where I know "A" uses a Sword and "B" uses a Lance? I can't directly join them by id and name because their value isn't exactly the same, but Weapon has a W-prefix on it.
I made some attempts with $substr but no success so far.
db.heroes.aggegate( [
{
$lookup: {
from: 'weapons',
let: { heroId: '$id' },
pipeline: [
{
$match: {
$expr: {
$eq: [ '$$heroId', { $substr: [ '$name', 1, -1 ] } ]
}
}
}
],
as: 'weapon'
}
}
] )
For reference, I also tried just hard-coding an ID with { $match: { $expr: { $eq: [ '$$heroId', 'HID_1' ] } } } and it didn't work. I could just rename all WHID to HID, but I am curious about whether it is possible or not.

Answer 1

Use $project to append the "W" to the heroID and then do a regular lookup like described here:
https://stackoverflow.com/a/46969468

Answer 2

I am laughing so hard right now, the query I posted is not the same I have in my code, and apparently I fixed it without knowing while I was copying it into the question. My let was wrong and defined weapons.name instead of heroes.id.
For anyone having a similar issue, the aggregate in the original post works as it should. I didn't notice it until #varman pointed it out, so thank you! And sorry for the silly mistake.

Answer 3

Try this...
db.heroes.aggregate([
{
$project: {
_id: 1,
name: 1,
newID: {
$concat: [
"W",
"$_id"
]
}
}
},
{
"$lookup": {
"from": "weapons",
localField: "newID",
foreignField: "name",
"as": "data"
}
},
{
$unwind: "$data"
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$data",
"$$ROOT"
]
}
}
},
{
$project: {
data: 0,
newID: 0
}
}
])
or
db.heroes.aggregate([
{
$lookup: {
from: "weapons",
let: {
heroId: "$id"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$$heroId",
{
$substr: [
"$name",
1,
-1
]
}
]
}
}
}
],
as: "data"
}
},
{
$unwind: "$data"
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$data",
"$$ROOT"
]
}
}
},
{
$project: {
data: 0
}
}
])
output:
[
{
"_id": "HID_1",
"name": "A",
"weapon": "Sword"
},
{
"_id": "HID_2",
"name": "B",
"weapon": "Lance"
}
]
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