Issue with Mongoose aggregation - node.js

Question

My requirement is for a particular user get all the expenses which are grouped based on the month and return the Month and the total expenses for that month.
I have collection something like below.
expenses = {{
"_id": {
"$oid": "5bc0f74df46dae0bf4ffdc39"
},
"user": {
"$oid": "5bab847a5b0d2e2ce8b4cbe5"
},
"date": "15-Mar-2018",
"expenseInfo": "Clothes",
"category": "Shopping",
"amount": 100,
"__v": 0
},
{
"_id": {
"$oid": "5bc0f78bf46dae0bf4ffdc3b"
},
"user": {
"$oid": "5bab847a5b0d2e2ce8b4cbe5"
},
"date": "11-Apr-2018",
"expenseInfo": "mobile",
"category": "Bills",
"amount": 100,
"__v": 0
},
{
"_id": {
"$oid": "5bc0f76cf46dae0bf4ffdc3a"
},
"user": {
"$oid": "5bab847a5b0d2e2ce8b4cbe5"
},
"date": "18-Apr-2018",
"expenseInfo": "Petrol",
"category": "Fuel",
"amount": 20,
"__v": 0
}
}
Here is my Expense schema.
Also, Note that user object(user is a different schema which is referred in Expense schema)
const ExpenseSchema = new mongoose.Schema({
user: {
type: mongoose.Schema.Types.ObjectId,
ref: 'user'
},
date: {
type: String
},
expenseInfo: {
type: String,
required: true
},
category: {
type: String,
required: true
},
amount: {
type: Number,
required: true
}
});
I tried something like this, but I am not getting any data. What is the issue here?
Expense.aggregate([{
$match: {
"_id": request.user.id
}
},
{
"$group": {
"_id": {
"$arrayElemAt": [{
"$split": ["$date", "-"]
}, 1]
},
"Total": {
"$sum": "$amount"
}
}
}
])