[{"_id":"Toystore A","toy_array":[{_id:1,"toy":"buzz"},{_id:2,"toy":"pope"}]},{"_id":"Toystore B","toy_array":[{_id:1,"toy":"jessie"}]}]

db.collection.aggregate([{$match:{"_id":"Toystore A"}},{$addFields:{toy_array:{$reduce:{input:[{_id:3,"toy":"woddy"},{_id:4,"toy":"buzz"}],initialValue:"$toy_array",in:{$cond:[{$in:["$$this.toy","$toy_array.toy"]},"$$value",{$concatArrays:[["$$this"],"$$value"]}]}}}}}])