How to get Child data after filtering dataset of Parents in a single mongo collection? - mongodb

Question

I have a single mongo collection called "CourseCollection" and it contains both parent and child doc. Any document with the key "Parent" is a child doc and a parent can have multiple child doc.
{
"_id" : "abracadavra",
"Name" : "abracadavra",
"Description" : "",
"Type" : "Spell",
"Parent" : {
"_id" : "Magic",
"Type" : "Course",
"Name" : "Magic"
}
},
{
"_id" : "Magic",
"Name" : "Magic",
"Type" : "Course",
"Access" : [
{
"_id" : "2sssdw5oe",
"Name" : "Abc"
},
{
"_id" : "4fddfye42",
"Name" : "Xyz"
}
]
}
What I'm trying to do is, based on the Access of Parent doc, I'm trying to get all the child doc.
Existing and working solution:
The solution that I have currently is to perform 2 queries.
Query 1. Get all the courses that the user has access to.
db.getCollection("CourseCollection").find({"Type": "Course", "Access._id": {"$in": ["2sssdw5oe"]}})
Query 2. Since I'm using Python, I do a list comprehension to get only the IDs of the course and then perform another query with this list
db.getCollection("CourseCollection").find({"Type": "Spell", "Parent._id": {"$in": course_list_id}})
Is there a way to get the child data after filtering out the parent in a single query. I also tried aggregation but only the results of the previous stage are passed to the next stage.

Answer 1

I guess you're trying to do something like this:
db.CourseCollection.aggregate([
{
"$match": {
"Type": "Spell"
}
},
{
"$lookup": {
"from": "CourseCollection",
"localField": "Parent._id",
"foreignField": "_id",
"as": "Parents"
}
},
{
"$match": {
"Parents": {
"$elemMatch": {
"Type": "Course",
"Access._id": {
"$in": [
"2sssdw5oe"
]
}
}
}
}
}
])
You can achieve the same result doing this too:
db.CourseCollection.aggregate([
{
"$match": {
"Type": "Spell"
}
},
{
"$lookup": {
"from": "CourseCollection",
"localField": "Parent._id",
"foreignField": "_id",
"as": "Parents",
"pipeline": [
{
"$match": {
"Type": "Course",
"Access._id": {
"$in": [
"2sssdw5oe"
]
}
}
}
]
}
},
{
"$match": {
"Parents.0": {
"$exists": true
}
}
}
])