db={"Book":[{_id:1,title:"Book 1",chapters:[1,2]},{_id:2,title:"Book 2",chapters:[2]}],Chapter:[{_id:1,title:"C TITLE 1",status:"unfinished",book:1},{_id:2,title:"C TITLE 2",status:"finished",book:1}]}

db.Book.aggregate([{"$lookup":{from:"Chapter",as:"chapters_list",let:{chapterIds:"$chapters"},pipeline:[{$match:{$expr:{$and:[{$in:["$_id","$$chapterIds"]},{$eq:["$status","unfinished"]}]}}}]}},{$match:{chapters_list:{$ne:[]}}},{$project:{chapters:1,title:1}}])