Multiple condition with $lookup aggregation - node.js

Question

I am looking at using best practices with the following problem with Mongoose.
I've got three schemas:
const SchemaA = new Schema({
name: String
});
const SchemaB = new Schema({
schemaA: {
type: Schema.Types.ObjectId,
ref: 'SchemaA',
required: true
}
});
const SchemaC = new Schema({
schemaB: {
type: Schema.Types.ObjectId,
ref: 'SchemaB'
},
user: {
type: Schema.Types.ObjectId,
ref: 'User'
}
});
I need to get schemaB objects by schemaA id with schemaC attached, but filtered by user.
getSchemaBs: async (schemaAId, userId) => {
try {
SchemaB.find({ schemaA: schemaAId }).populate('schemaC': where user === userId);
} catch (e) {
throw new Error('An error occurred while getting schemasB for specified schemaA.');
};
I'm in the middle of refactoring of the code written with use of Mongo native driver for NodeJS. Now I want to make it simpler with Mongoose usage.
Earlier version of the code (keep in mind it could not follow best practices):
getList: function (schemaAId, userId) {
return new Promise(
function (resolve, reject) {
db.collection('schemaB').aggregate([{
$match: {
'isDeleted': false,
'schemaA': ObjectID(schemaAId)
}
},
{
$lookup: {
from: "schemaC",
localField: "_id",
foreignField: "schemaBId",
as: "schemasC"
},
},
{
$project: {
_id: true,
schemaAId: true,
// other neccessary fields with true (several lines - makes code ugly and messy)
schemasC: {
"$arrayElemAt": [{
$filter: {
input: "$schamasC",
as: "schemaC",
cond: {
$eq: ["$$schemaC.userId", ObjectID(userId)]
}
}
}, 0]
}
}
}
]).toArray(function (error, result) {
if (error) {
reject(error);
} else {
resolve(result);
};
});
});
}
How can I deal with that the best way?

Answer 1

What you are trying to do can be better done by using mongodb 3.6 $lookup syntax which filters the document inside the $lookup pipeline
db.collection('schemaB').aggregate([
{ "$match": { "isDeleted": false, "schemaA": ObjectID(schemaAId) }},
{ "$lookup": {
"from": "schemaC",
"let": { "schemaBId": "$_id" },
"pipeline": [
{ "$match": {
"$expr": { "$eq": ["$schemaBId", "$$schemaBId"] },
"userId": ObjectID("5b5c747d8209982630bbffe5")
}}
],
"as": "schemasC"
}}
])