Mongo DB - query nested arrays of objects in referenced document - mongodb

Question

I have few referenced collections: Employees, Roles, Schools...
I need to query the database, to list all employees that are employed by a certain school. That means all Employees, that has certain School ID in their corresponding role document must be returned in the list or array of results.
So far I have tried to find it like this:
const employees = mongoose.schema("Employees");
employees.find({})
.populate({
"path": "roles"
})
.then(function(docs){
docs.filter(function(){
// write my filters here....
})
});
but this still is inefficient and I can't make it work.
It has to be a smarter way...
This is my Employee document, which references a document in Roles Collection:
{
"_id" : { "$oid" : "57027f1b9522d363243abr42" },
"assignedRoles": [
{
"type": "teacher",
"schools": [
{
"schoolId": "57027f1b9522d3632423rf32",
"classes" : ["math", "science"],
"active": true
},
{
"schoolId": "57027f1b9522d36324252fs2",
"classes" : ["science"],
"active": true
},
{
"schoolId": "57027f1b9522d36324242f35",
"classes" : ["math"],
"active": true
},
]
},
{
"type": "manager",
"schools": [
{
"schoolId": "57027f1b9522d3632423rf32",
"active": true
},
{
"schoolId": "57027f1b9522d36324252fs2",
"active": true
}
]
}
{
"type": "principal",
"schools": [
{
"schoolId": "57027f1b9522d3632423rf32",
"active": true
}
]
}
],
"rolesMeta": "Meta Info for the Roles"
}
The following is the list of Schools - irrelevant for the task, I am just adding that for the completion:
{
"_id": { "$oid" : "57027f1b9522d3632423rf32" },
"name": "G.Washington HS",
"district": "six",
"state": "New Mexico"
},
{
"_id": { "$oid" : "57027f1b9522d36324252fs2" },
"name": "A. Lincoln HS",
"district": "six",
"state": "New Mexico"
},
{
"_id": { "$oid" : "57027f1b9522d36324242f35" },
"name": "T. Roosvelt HS",
"district": "four",
"state": "New Mexico"
}

Answer 1

The Resolution to the problem is to use MongoDB Aggregate framework to be able to drill down and "flatten" the array structure.
Here is how I have resolved it:
db.employees.aggregate([
// I use $project to specify which field I want selected from the collection
{
$project: {_id: 1, "roles": 1}
},
// $lookup helps me get results and populate the 'role' field from the Roles collection. This returns an array
{
$lookup: {
"from": "roles",
"localField": "roles",
"foreignField": "_id",
"as": "roles"
}
},
// $unwind is self explanatory - it flattens the array and creates multiple objects, instances of the parent object for each array element.
{
$unwind: "$roles"
},
// now repeat for each array in this document
{
$unwind: "$roles.assignedRoles"
},
// repeat again, since I need to look in the schools array.
{
$unwind: "$roles.assignedRoles.schools"
},
// The following line will return all records that the school ID checks out.
{
$match: {"roles.assignedRoles.schools.schoolId": school}
},
// $group is a necessary cleanup task so we avoid repeating employee records... Nicely Done! :)
{
$group: {
"_id": "$_id",
"roles": {"$push": "$roles"},
}
}
])
.exec()
.then(function (docs) {
// here you process the resulted docs.
});