db={"computer":[{_id:ObjectId("6098c5ab615d9e23543d0f6f"),name:"TestComputer",category:"A",tags:["TestTag"]}],"settings":[{_id:ObjectId("61e56339b528bf009feca149"),name:"FirstSetting",category:"A",priority:1,tags:[ObjectId("61b873ec6d075801f7a97e18")]},{_id:ObjectId("61e56339b528bf009feca150"),name:"SecondSetting",category:"A",priority:2,tags:[ObjectId("61b873ec6d075801f7a97e18")]}]}

db.computer.aggregate([{$match:{_id:ObjectId("6098c5ab615d9e23543d0f6f")}},{"$lookup":{"from":"settings",localField:"category","foreignField":"category","pipeline":[{$sort:{priority:1}},{$limit:1},{$project:{_id:1}}],"as":"activeSetting"}},{$set:{activeSetting:{$first:"$activeSetting._id"}}}])