MongoDb: add element to array if not exists

I'm using node.js and Mongo.db (I'm newly on Mongo). I have a document like this:

Tag : {
   name: string,
   videoIDs: array
}

The idea is, the server receives a JSON like

JSON: { 
    name: "sport",
    videoId: "34f54e34c"
}

with this JSON, it has to find the tag with the same name and check if in the array it has the videoId, if not, insert it to the array.

How can I check the array and append data?

You can use $addToSet operator to check exist before append element into array.

db.tags.update(
    {name: 'sport'},
    {$addToSet: { videoIDs: "34f54e34c" } }
);

In this update statement example, mongoDB will find the TAG document which matches name == sport, and then check whether the videoIDs array contains 34f54e34c. If not, append it to the array.

Detail usage of $addToSet please read here.

I didn't test it, but you can try something like:

yourDb.find({ name: "sport", 
              videoIDs: { $in: ["34f54e34c"] } })
      .update({$addToSet: { videoIDs: "34f54e34c" }});

If you need how: MongoDb in nodeJs implementation, you can start with:

http://jsfiddle.net/1ku0z1a1/

$addToSet operator check for the empty or existing array in document.

 db.col_name.update({name :"name_for_match"},{$addToSet :  { videoId : "video_id_value"}})