mongoose aggregation $group with nested object - mongodb

Question

I have the following resolver:
const result = await UserPassage.aggregate([
{ $sort: { createdAt: -1 } },
{
$group: {
_id: '$level',
level: { $first: '$level' },
passageId: { $first: '$passageId' },
userId: { $first: '$userId' },
type: { $first: '$type' },
category: { $first: '$category' },
score: { $first: '$score' },
completedStage: { $first: '$completedStage' },
userPassageStatsId: {
_id: { $first: '$_id' },
stats: {
readingTime: { $first: '$readingTime' },
qtdVocab: { $first: '$qtdVocab' },
qtdTestDone: { $first: '$qtdTestDone' },
totalQuiz: { $first: '$totalQuiz' },
progress: { $first: '$progress' },
},
},
},
},
{ $sort: { level: 1 } },
]);
await UserPassageStats.populate(result, { path: 'userPassageStatsId' });
The problem is that I need to populate 'userPassageStatsId' and return it but it's not working well returning the following error:
MongoError: The field 'userPassageStatsId' must be an accumulator object
does anyone knows what I am doing wrong?

Answer 1

$group can only contain _id or accumulator objects like $first, $last, $sum etc. In your case your building nested object and that syntax is not allowed - accumulator has to be on a top level. You can try two approaches, either return flat structure from $group and then reshape using $project:
{
$group: {
_id: '$level',
level: { $first: '$level' },
passageId: { $first: '$passageId' },
userId: { $first: '$userId' },
type: { $first: '$type' },
category: { $first: '$category' },
score: { $first: '$score' },
completedStage: { $first: '$completedStage' },
userPassageStatsId_id: { $first: '$_id' },
readingTime: { $first: '$readingTime' },
qtdVocab: { $first: '$qtdVocab' },
qtdTestDone: { $first: '$qtdTestDone' },
totalQuiz: { $first: '$totalQuiz' },
progress: { $first: '$progress' }
}
},
{
$project: {
_id: 1,
level: 1,
...,
userPassageStatsId: {
_id: "$userPassageStatsId_id",
stats: {
readingTime: "$readingTime",
...
}
}
}
}
or use $$ROOT to capture first object for every group and reshape it using $project:
{
$group: {
_id: '$level',
d: { $first: "$$ROOT" }
}
},
{
$project: {
_id: 1,
level: "$d.level",
...,
userPassageStatsId: {
_id: "$d._id",
stats: {
readingTime: "$d.readingTime",
...
}
}
}
}