Database
db={"book":[{_id:1,title:"book1",chapters:[1,2]},{_id:2,title:"book2",chapters:[3,4]}],chapter:[{_id:1,title:"b1c1",status:"unfinished",book:1},{_id:2,title:"b1c2",status:"finished",book:1},{_id:3,title:"b2c1",status:"unfinished",book:2},{_id:4,title:"b2c2",status:"unfinished",book:2}]}
Query
db.book.aggregate([{$unwind:"$chapters",},{$lookup:{from:"chapter",localField:"chapters",foreignField:"_id",as:"chapter",},},{$match:{"chapter.status":"unfinished",},},{$group:{_id:"$_id",title:{$first:"$title"},},}])