Aggregate Query returns null array - mongodb

Question

I have three collections in EmployeeDB.
Employee
{
"user_id" : "EMP001",
"FirstName" : "Manoj",
"LastName" : "sharma",
"Status" : "0/1",
"CreateDate" : "1988-10-11T18:30:00.000Z"
}
EmpContact
{
"user_id" : "EMP001",
"Phone" : "9999999999",
"Email" : "xyz#gmail.com"
}
EmpInfo
{
"user_id" : "EMP001",
"Gender" : "Male",
"Age" : NumberInt(20),
"Designation" : "manager",
"Salary" : "5000"
}
I need to group the three collections and display all fields.
I tried the below code,
db.Employee.aggregate([
{$lookup: {from: "EmpContact",localField: "user_id",foreignField: "user_id",as: "contact"}},
{$unwind:"$EmpContact"},
{$lookup: {from: "EmpInfo",localField: "user_id",foreignField: "user_id",as: "empinfo"}},
{$unwind:"$EmpInfo"},
{
$match:{user_id : "EMP001"}
}
]).toArray();
But I got an empty array, Please help me...

Answer 1

The problem comes from your $unwind.
Since you name your field as contact in the $lookup, the field EmpContact doesn't exist.
The same thing for the second unwind, EmpInfo doesn't exist but empinfo does
Here is the full query
db.Employee.aggregate([
{
$lookup: {
from: "EmpContact",
localField: "user_id",
foreignField: "user_id",
as: "contact"
}
},
{
$unwind: "$contact"
},
{
$lookup: {
from: "EmpContact",
localField: "user_id",
foreignField: "user_id",
as: "empinfo"
}
},
{
$unwind: "$empinfo"
},
{
$match: {
user_id: "EMP001"
}
}
])
Try it here