MongoDB multiple Lookup into same collection - sql

Question

I have two collections Bill and Employee. Bill contains the information about the monthly student bill and Employee contains all types of people working in the school (Accountant, Teachers, Maintenance etc).
Bill has billVerifyBy and classteacher field which points to the records of Employees.
Bill collection
{
"_id": ObjectId("ab12dns..."), //mongoid
"studentname": "demoUser",
"class": { "section": "A"},
"billVerifiedBy": "121212",
"classteacher": "134239",
}
Employee collection
{
"_id": ObjectId("121212"), // random number
"name": "Darn Morphy",
"department": "Accounts",
"email": "dantest#test.com",
}
{
"_id": ObjectId("134239"),
"name": "Derreck",
"department": "Faculty",
"email": "derrect145#test.com",
}
I need to retrieve the Accounts and Teacher information related to a particular bill. I am using Mongodb lookup to get the information. However, I have to lookup to the same table twice since billVerifiedBy and classteacher belong to the same Employee tables as given below.
db.bill.aggregate([
{
$lookup: {"from": "employee", "localField": "billVerifiedBy", "foreignField": "_id", "as": "accounts"}},
},
{
$lookup: {"from": "employee", "localField": "classteacher", "foreignField": "_id", "as": "faculty"}},
},
{
$project: {
"studentname": 1,
"class": 1,
"verifiedUser": "$accounts.name",
"verifiedByEmail":"$accounts.email",
"facultyName": "$faculty.name",
"facultyEmail": "$faculty.email"
}
}
]
I don't know if this is the good way of arranging the Accounts and Faculty information in the single Employee collection. And is it right thing to lookup twice with same collection. Or should I create separate Accounts and Faculty collection and lookup with it. Please suggest what would be the best approach in terms of performance.

Answer 1

In mongodb, when you want to join multiple documents from the same collection, you can use "$lookup" with its "pipeline" and "let" options. It filters documents that you want to take with defined variables.
db.getCollection('Bill').aggregate([{
"$lookup": {
"as": "lookupUsers",
"from": "Employee",
// define variables that you need to use in pipeline to filter documents
"let": {
"verifier": "$billVerifiedBy",
"teacher": "$classteacher"
},
"pipeline": [{ // filter employees who you need to filter.
"$match": {
"$expr": {
"$or": [{
"$eq": ["$_id", "$$verifier"]
},
{
"$eq": ["$_id", "$$teacher"]
}
]
}
}
},
{ // combine filtered 2 documents in an employee array
"$group": {
"_id": "",
"employee": {
"$addToSet": {
"_id": "$_id",
"name": "$name",
"department": "$department",
"email": "$email"
}
}
}
},
{ // takes item from the array by predefined variable.
"$project": {
"_id": 0,
"billVerifiedBy": {
"$slice": [{
"$filter": {
"input": "$employee",
"cond": {
"$eq": ["$$this._id", "$$verifier"]
}
}
},
1
]
},
"classteacher": {
"$slice": [{
"$filter": {
"input": "$employee",
"cond": {
"$eq": ["$$this._id", "$$teacher"]
}
}
},
1
]
}
}
},
{
"$unwind": "$billVerifiedBy"
},
{
"$unwind": "$classteacher"
},
]
}
},
{
"$unwind": "$lookupUsers"
},
]);
Output is like that:
{
"_id": ObjectId("602916dcf4450742cdebe38d"),
"studentname": "demoUser",
"class": {
"section": "A"
},
"billVerifiedBy": ObjectId("6029172e9ea6c9d4776517ce"),
"classteacher": ObjectId("6029172e9ea6c9d4776517cf"),
"lookupUsers": {
"billVerifiedBy": {
"_id": ObjectId("6029172e9ea6c9d4776517ce"),
"name": "Darn Morphy",
"department": "Accounts",
"email": "dantest#test.com"
},
"classteacher": {
"_id": ObjectId("6029172e9ea6c9d4776517cf"),
"name": "Derreck",
"department": "Faculty",
"email": "derrect145#test.com"
}
}
}