You must think, like in relational database but within models. So you can set relation like one to one, one to many..
Take a look at Media.Occurrence that it is similar:
```javascript
  319 Media.Occurrence = Backbone.RelationalModel.extend({
  320     urlRoot: 'occur',
  321     idAttribute: '_id',
  322     relations: [{
  323         includeInJSON: '_id',
  324         type: Backbone.HasOne,
  325         key: 'playlist',
  326         relatedModel: 'Media.Playlist',
  327         reverseRelation: {
  328             key: 'occurrences',
  329             collectionType: 'Media.Schedule',
  330             includeInJSON: '_id',
  331             type: Backbone.HasMany
  332         }
  333     }],
...
  376 });
```