[{"_id":1,"books":[{"id":2},{"id":3,"pictures":[]}]}]

db.collection.update({},[{"$set":{"books":{"$map":{"input":"$books","in":{"$cond":[{"$eq":["$$book.id",2]},{"$mergeObjects":["$$book",{"pictures":{"$concatArrays":[{"$cond":[{"$isArray":["$$book.pictures"]},"$$book.pictures",[]]},[{"description":"nice picture"}]]}}]},"$$book"]},"as":"book"}}}}])