db={"orders":[{"_id":ObjectId("5f242e68ceebdd59a456fee5"),"date":ISODate("2020-07-30T22:00:00.000Z"),"user":{"ref":"user","id":"user_0"},"item":"banana"}],"users":[{"_id":"user_0","login":"login_0","password":"$2a$10$mE.qmcV0mFU5NcKh73TZx.z4ueI/.bDWbj0T1BYyqP481kGGarKLG","first_name":"John","last_name":"Kennedy","email":"jfk@gmail.com","activated":true}]}

db.users.aggregate([{"$lookup":{"from":"orders","localField":"_id","foreignField":"user.id","as":"users"}},{$project:{_id:0,first_name:1,last_name:1,nbOfOrders:{$size:"$users"}}}])