Database
db={"chapters":[{"_id":1,"name":"First chapter","questions":[{text:"What is life?",answer:"42",},{text:"What is my name?",answer:"uwewewe osas",}],},],"notes":[{"_id":1,"chapter":1,"question":1,"content":"Some important note regarding this question",}]}
Query
db.notes.aggregate([{$lookup:{from:"chapters",let:{chapter_id:"$chapter"},pipeline:[{$match:{$expr:{$eq:["$_id","$$chapter_id"]}}},{$project:{_id:1,name:1,questions:1,},},],as:"chapter",},},{$addFields:{chapter:{$first:"$chapter",},},},{$addFields:{question:{$arrayElemAt:["$chapter.questions","$question"],},},},{$project:{"chapter.questions":0,},},])