Finding parent documents based on a child ID - mongodb

Question

Suppose I have the following schemas
let TutorialQuizSchema = new mongoose.Schema({
groups: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Group' }]
});
let GroupSchema = new mongoose.Schema({
members: [{ type: mongoose.Schema.Types.ObjectId, ref: 'User' }],
responses: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Response' }]
});
let UserSchema = new mongoose.Schema({
name: {
first: String,
last: String
}
});
Given a user ID, is it possible to query all the tutorial quizzes with all the groups that have the user as one of its members?
I'm new to aggregation, but I think it would be something like this
TutorialQuiz.aggreggate([
{
$unwind: '$groups'
},
{
$lookup: {
from: 'groups',
localField: 'groups',
foreignField: '_id',
as: 'group'
}
},
{
$unwind: '$group'
},
{
$match: {
'group.members': { $in: [req.user._id] }
}
}
]).exec((err, data) => {
// etc
})
If I am correct, my only problem with this is that the data comes out flattened. Is it possible to unflatten it to maintain the hierarchical structure (like if we were just doing a find + populate query) ?
Note: if there is better/easier way to do this, I am open to suggestions also.

Answer 1

You can $lookup objectIds directly without $unwinding and use $filter instead of second $unwind inside the $addFields stage to filter the group on presence of user_id value in the members array in 3.4 version.
Something like
TutorialQuiz.aggreggate([
{
$lookup: {
from: 'groups',
localField: 'groups',
foreignField: '_id',
as: 'group'
}
},
{
$addFields: {
group: {
$filter: {
input: "$group",
as: "group",
cond: { $in: [ req.user._id, "$$group.members" ] }
}
}
}
}
])