MongoDB - Apply similar lookup on multiple columns in main table - mongodb

Question

I'm a newbie to MongoDB, I'm trying to aggregate complete details of students in referencing with other collections.
students collection structure:
{
"_id" : ObjectId("5cc973dd008221192148177a"),
"name" : "James Paulson",
"teachers" : [
ObjectId("5cc973dd008221192148176f"),
ObjectId("5cc973dd0082211921481770")
],
"attenders": [
ObjectId("5cc973dd0082211921481732"),
ObjectId("5cc973dd008221192148173f")
]
}
staff collection structure:
{
"_id" : ObjectId("5cc973dd008221192148176f"),
"name" : "John Paul",
"subject" : [
"english",
"maths"
]
}
{
"_id" : ObjectId("5cc973dd0082211921481771"),
"name" : "Pattrick",
"subject" : [
"physics",
"history"
]
}
{
"_id" : ObjectId("5cc973dd0082211921481732"),
"name" : "Roger",
"subject" : [
"sweeper"
]
}
{
"_id" : ObjectId("5cc973dd008221192148173f"),
"name" : "Ken",
"subject" : [
"dentist"
]
}
This is the query I used for the retrieval of all teacher details of a particular student.
Query:
db.getCollection('students').aggregate([
{
$unwind: "$teachers"
},
{
$lookup:
{
from: 'staff',
localField: 'teachers',
foreignField: '_id',
as: 'teachers'
}
}
]);
Result:
{
"_id" : ObjectId("5cc973dd008221192148177a"),
"name" : "James Paulson",
"teachers" : [
{
"_id" : ObjectId("5cc973dd008221192148176f"),
"name" : "John Paul",
"subject" : [
"english",
"maths"
]
},
{
"_id" : ObjectId("5cc973dd008221192148176f"),
"name" : "Pattrick",
"subject" : [
"physics",
"history"
]
}
],
"attenders": [
ObjectId("5cc973dd0082211921481732"),
ObjectId("5cc973dd008221192148173f")
]
}
As you can see, the attenders array is also similar to teachers except the difference in column name in students table. So how to applying a similar query to the second column (attenders)? Also is there any way to select specific columns from the second table (like only _id and name from staff collection)?
Any help on this would be greatly appreciated.

Answer 1

You can use below aggregation with mongodb 3.6 and above
Firstly you don't need to use $unwind here as your field already contains array of ObjectIds. And to select the specific field from the referenced collection you can use the custom $lookup with pipeline and $project the fields inside it.
db.getCollection('students').aggregate([
{ "$lookup": {
"from": "staff",
"let": { "teachers": "$teachers" },
"pipeline": [
{ "$match": { "$expr": { "$in": [ "$_id", "$$teachers" ] } } }
{ "$project": { "name": 1 }}
],
"as": "teachers"
}},
{ "$lookup": {
"from": "attenders",
"let": { "attenders": "$attenders" },
"pipeline": [
{ "$match": { "$expr": { "$in": [ "$_id", "$$attenders" ] } } }
],
"as": "attenders"
}}
])