How do I join collections based on a substring in MongoDB - mongodb

Question

Just trying to wrap my head around how to reference a collection based on a substring match.
Say I have one collection with chat information (including cell_number: 10 digits). I have another collection in which every document includes a field for area code (3 digits) and some other information about that area code, like so:
Area codes collection excerpt:
{
"_id" : ObjectId("62cf3d56580efcedf7c3b845"),
"code" : "206",
"region" : "WA",
"abbr" : "WA"
}
{
"_id" : ObjectId("62cf3d56580efcedf7c3b846"),
"code" : "220",
"region" : "OH",
"abbr" : "OH"
}
{
"_id" : ObjectId("62cf3d56580efcedf7c3b847"),
"code" : "226",
"region" : "Ontario",
"abbr" : "ON"
}
What I want to do is be able to grab out just the document from the area codes collection which has a "code" field value matching the first 3 characters of a cell number. A simple example is below. Note that in this example I have hardcoded the cell number for simplicity, but in reality it would be coming from the chat_conversations "parent" collection:
db.chat_conversations.aggregate([
{$match: {wait_start: {$gte: new Date("2020-07-01"), $lt: new Date("2022-07-12")}}},
{$lookup: {
from: "area_codes",
let: {
areaCode: "$code",
cellNumber: "2065551234"
},
pipeline: [
{$match: {
$expr: {
$eq: ["$$areaCode", {$substr: ["$$cellNumber", 0, 3]}]
}
}}
],
as: "area_code"
}},
]).pretty()
Unfortunately nothing I try here seems to work and I just get back an empty array from the area codes collection.
What am I missing here?

Answer 1

you should do a let on chat_converstion
or you should use let with vars and in expressions
this query should work for you
db.chat_conversations.aggregate([
{$match: {wait_start: {$gte: new Date("2020-07-01"), $lt: new Date("2022-07-12")}}},
{
$lookup: {
from: "area_codes",
let:{
"cell_Number":{$substr:["$cellNumber",0,3]}
},
pipeline:[
{
$match:{
$expr:{
$eq: [ "$code", "$$cell_Number" ]
}
}
}],
as: "data"
}
},
])