How to give a mongoose method to a model?

Question

MongoDB

Let's say there is a minimal mongo model

var Schema = mongoose.Schema;

var userSchema = new Schema({
name: String
})

userShema.methods.say = function(){
console.log('hello')
}

var User = mongoose.model('users', userSchema);

module.exports = User;

But when I try to access the method from the client, I get an error that there is no such method (this.model.say is not a function)
And when even in the browser console model (this.model) there is no say method anywhere, neither in obj nor in it __proto__ .
I suspect that the problem is in my misunderstanding of the work of callbacks. Please explain in simple terms
Thank you