Unique array values in Mongoose

Take a look at the Mongo documentation on the $addToSet operator.

To my knowledge, the only way to do this in mongoose is to call the underlying Mongo operator (mentioned by danmactough). In mongoose, that'd look like:

var idToUpdate, theIdToAdd; /* set elsewhere */
Model.update({ _id: idToUpdate }, 
             { $addToSet: { theModelsArray: theIdToAdd } }, 
             function(err) { /*...*/ }
);

Note: this functionality requires mongoose version >= 2.2.2