Mongoose aggregation: Query within "group" phase
You can use $lookup with pipeline,
$lookupto joinAssignmentcollection$matchassignment id and is public true$matchoutside lookup, check is assignment not empty [],
{ $match: ... }, // skipping // add this { $lookup: { from: "Assignment", let: { aId: "$assignment" }, pipeline: [ { $match: { $expr: { $eq: ["$_id", "$$aId"] }, isPublic: true } } ], as: "assignments" } }, // add this { $match: { assignments: { $ne: [] } } }, { $group: ... }, // skipping { $sort: ... } // skipping