Proper Mongoose Subdocument Relation
You can try below aggregation
Group.aggregate([ { "$lookup": { "from": Invitations.collection.name, "let": { "invitations": "$invitations" }, "pipeline": [ { "$match": { "$expr": { "$in": [ "$_id", "$$invitations" ] } } } ], "as": "invitations" }}, { "$match": { "invitations.user": user._id } }])or this
Group.aggregate([ { "$lookup": { "from": Invitations.collection.name, "let": { "invitations": "$invitations" }, "pipeline": [ { "$match": { "user": user._id, "$expr": { "$in": [ "$_id", "$$invitations" ] } }} ], "as": "invitations" }}, { "$match": { "invitations.user": user._id } }])And the last one which I think is the best option to start with User collection
User.aggregate([ { "$match": { "_id": user._id }}, { "$lookup": { "from": Groups.collection.name, "let": { "userId": "$_id" }, "pipeline": [ { "$match": { "$expr": { "$eq": [ "$created_by", "$$userId" ] }}} ], "as": "groups" }}])