Question

When creating new collection (Choices) I want to set a property on it (ex: _question) which links back to the containing Model (MultipleChoiceQuestion)

Answer 1

I actually found that although my 1st answer technically works, there’s a plugin that makes care of storing collections in models (and creating appropriate One->Many, One->One and Many->One relationships

https://github.com/PaulUithol/Backbone-relational

Using that plugin you store the parent question as an ATTRIBUTE

m.MultipleChoiceQuestion = Backbone.RelationalModel.extend({
    relations: [{
        type: Backbone.HasMany,
        key: 'choices', //says to store the collection in the choices attribute
        relatedModel: m.Choice, //knows we are talking about the choices models
        includeInJSON: true, //when we do toJSON() on the Question we want to show serialize the choices fully
        reverseRelation: {
            key: 'question', //in the CHOICE object there will be an attribute called question which contains a reference to the question 
            includeInJSON: false //when we do .toJSON on a CHOICE we don't want to show the question object (or maybe we want to show the ID property in which case we set it to "id")
        }
    }],
    coolFunction: function () {
        this.get('choices').each(function(choice){
            choice.doSomethingChoicey();
        });
    }
});

So now if we are in the choices model we can fully reference anything in the parent question:

m.Choice = m.RelationalModel.extend({
    coolFunction: function(){
        this.get('question').doSomemethingQuestiony();
        var question_numer = this.get('question').get('question_number');
    }
});