db={"coll1":[{"_id":"shalom","arr":[1,2,3]}],"coll2":[{"_id":"shalom","arr":[4,5,6]}]}

db.coll1.aggregate([{"$lookup":{"from":"coll2","localField":"_id","foreignField":"_id","as":"newArray"}},{"$set":{"newArray":{"$arrayElemAt":["$newArray",0]}}},{"$set":{"arr":{"$setUnion":["$arr","$newArray.arr"]}}},{"$project":{"newArray":0}}])