How to create mongodb aggregation pipeline between two collections? - node.js

Question

I want to create a Mongodb aggregation pipeline for a collection named Transaction.
The Transaction collection has values amount, categoryID, description and I also have a Category collection with values type, icon and color.
I want the pipeline to show the top3 categories with their percentage values and a others category with its percentage value.
the transaction type should be Expense which it should get from the Category collection and it should show all transactions having category type Expense. The top3 should then give the results as transaction with category (example)
type : Rent
percentage:45
type: Entertainment
percentage: 30
type: Food
percentage: 20
type: Others
percentage: 5
I tried it with Category collection but I don't want category to store amount, but Transaction should store amount.
Category.aggregate([
{
$match: {
type: 'expense'
}
},
{
$group: {
_id: "$name",
amount: { $sum: "$amount" }
}
},
{
$group: {
_id: null,
totalExpense: { $sum: "$amount" },
categories: {
$push: {
name: "$_id",
amount: "$amount"
}
}
}
},
{
$project: {
_id: 0,
categories: {
$map: {
input: "$categories",
as: "category",
in: {
name: "$$category.name",
percent: { $multiply: [{ $divide: ["$$category.amount", "$totalExpense"] }, 100] }
}
}
}
}
},
{
$unwind: "$categories"
},
{
$sort: { "categories.percent": -1 }
},
{
$limit: 3
}
])
This was the pipeline I used for it.
//edit
Tried the method suggested by Joe
Transaction.aggregate([
// Join the Transaction collection with the Category collection
{
$lookup: {
from: 'Category',
localField: 'categoryID',
foreignField: '_id',
as: 'category',
},
},
// Unwind the category array to separate documents
{
$unwind: '$category',
},
// Filter for transactions where the category type is "Expense"
{
$match: {
'category.type': 'Expense',
},
},
// Group transactions by category type and calculate the percentage
{
$group: {
_id: '$category.type',
total: { $sum: '$amount' },
count: { $sum: 1 },
},
},
{
$project: {
_id: 0,
category: '$_id',
percentage: {
$multiply: [{ $divide: ['$count', { $sum: '$count' }] }, 100],
},
},
},
// Sort the categories by percentage in descending order
{
$sort: { percentage: -1 },
},
// Limit the result to top 3 categories
{
$limit: 3,
},
// group the rest of the categories as others
{
$group: {
_id: null,
top3: { $push: '$$ROOT' },
others: { $sum: { $subtract: [100, { $sum: '$top3.percentage' }] } },
},
},
{
$project: {
top3: 1,
others: { category: 'Others', percentage: '$others' },
},
},
]);
I am getting an empty array rather than the values. I have data in the collections with the correct ID's. What might be the issue?
//Answer
This aggregation worked for me
Transaction.aggregate([
{
$match: {
userID: { $eq: UserID },
type: 'Expense',
},
},
{
$addFields: { categoryID: { $toObjectId: '$categoryID' } },
},
{
$lookup: {
from: 'categories',
localField: 'categoryID',
foreignField: '_id',
as: 'category_info',
},
},
{
$unwind: '$category_info',
},
{
$group: {
_id: '$category_info.name',
amount: { $sum: '$amount' },
},
},
{
$sort: {
amount: -1,
},
},
{
$group: {
_id: null,
total: { $sum: '$amount' },
data: { $push: '$$ROOT' },
},
},
{
$project: {
results: {
$map: {
input: {
$slice: ['$data', 3],
},
in: {
category: '$$this._id',
percentage: {
$round: {
$multiply: [{ $divide: ['$$this.amount', '$total'] }, 100],
},
},
},
},
},
others: {
$cond: {
if: { $gt: [{ $size: '$data' }, 3] },
then: {
amount: {
$subtract: [
'$total',
{
$sum: {
$slice: ['$data.amount', 3],
},
},
],
},
percentage: {
$round: {
$multiply: [
{
$divide: [
{
$subtract: [
'$total',
{ $sum: { $slice: ['$data.amount', 3] } },
],
},
'$total',
],
},
100,
],
},
},
},
else: {
amount: null,
percentage: null,
},
},
},
},
},
]);