how to use $regex search in referenced field in mongodb - node.js

Question

i am struggling with a task, which it is I used two schemas
User Schema
{
"first_name": String,
"last_name":String,
"address": String
}
Employee schema
{
user:{
type: ObjectId,
ref: 'User'
},
gross_pay: String,
net_pay: String
tax: String,
}
Well, how can I search first_name using $regex in Employee Schema in this user referenced field? I tried in so many ways, can't get it. how can i resolve it? Thanks in advance

Answer 1

First Approach:
Using $lookup aggregation
Employee.aggregate([
{ "$lookup": {
"from": "users",
"let": { "user": "$user" },
"pipeline": [
{ "$match": { "$expr": { "$eq": ["$_id", "$$user"] }}},
{ "$project": { "firstName": 1 }}
],
"as": "user"
}},
{ "$unwind": "$user" },
{ "$match": { "user.firstName": { "$regex": your_string, "$options": "i" }}}
])
But it is not the better approach as the $lookup stage is being applied to all the Employee documents and then $match to the users firstName makes the query bit slow.
Second Approach:
First find the users having firstName equal to the match string using $regex and then find the _id in the Employee collection.
const userIds = await (Users.find({ "firstName": { "$regex": your_string, "$options": "i" } })).map(user => user._id)
const employees = await Employee.find({ "user": { "$in": userIds }})
Third Approach:
Keep the single key firstName of the user schema in the employee schema
Employee schema
user: { type: ObjectId, ref: 'User' },
gross_pay: String,
net_pay: String
tax: String
firstName: String
and then use query directly
const userIds = await Employee.find({
"firstName": { "$regex": your_string, "$options": "i" }
})