Mongoose: use plugin in Schema static method

I use mongoose random plugin.
In my schema definition i call

GameSchema.plugin(random, { path: 'r' });  

After that I have a custom static method who use the plugin:

GameSchema.statics.someMethod {  
    [...]
    GameSchema.findRandom...  

And I get the error

TypeError: Object #<Schema> has no method 'findRandom'

Is there a way to achieve what I am trying to do or should I implement some kind of repository ?

EDIT:
Ben's answer worked, I needed to use findRandom on the model and not the schema.
Precision for those in my case: you need to declare first your static function

GameSchema.statics.someMethod {  
    [...]
    Game.findRandom...

then register your schema

var Game = mongoose.model('Game', GameSchema);

otherwise you'll get "Model .... has no method 'someMethod'"
Game variable in the static function is recognized event though it is defined only later in the script.
=> bonus question: does anyone know why it works ?

You're calling the method on the schema, whereas you need to be calling it on the model.

var Game = mongoose.model('Game', GameSchema);

Game.findRandom()...