db={"series":[{"_id":"1","name":"serie1","book genre":"Horror"},{"_id":"2","name":"serie2","book genre":"Romance"},{"_id":"3","name":"serie3","book genre":"Horror"},{"_id":"4","name":"serie4","book genre":"Horror"},],"books":[{"_id":"1","name":"book1","lenght":100,"serie":"serie1"},{"_id":"2","name":"book2","lenght":10,"serie":"serie3"},{"_id":"3","name":"book3","lenght":900,"serie":"serie3"},{"_id":"4","name":"book4","lenght":300,"serie":"serie1"},]}

db.series.aggregate([{$lookup:{from:"books",localField:"name",foreignField:"serie",as:"books"}},{$project:{_id:1,name:1,"book genre":1,avg:{"$ifNull":[{$avg:{$map:{input:"$books",in:"$$this.lenght"}}},0]}}}])