wondering about top-level persistence of a netsted object in backbone

how_to_save_this.js

TextCollection = Backbone.Collection.extend({
  model: Text,
  localStorage: new Backbone.LocalStorage('textCollection') // i only want to persist here
})

Text = Backbone.Model.extend({
  initialize: function(options){
    this.set('sentenceCollection', new SentenceCollection())
  }
})

Sentence = Backbone.Model.extend({})
  
SentenceCollection = Backbone.Collection.extend({
  model: Sentence,
  // I don't want a url or localStorage here, I just want to hand it up to the text 
  // should i override Sentence.save? how?
})