How do I declare a `type` key on a polymorphic relationship in Ember.js?

I'm trying to learn ember.js and I have a problem with trying to use the DS.FixtureAdapter with mock data. Whenever I try to access the store I get an Ember error of:

Error while processing route: exercises Assertion Failed: Ember Data expected a number or string to represent the record(s) in the pictureExercises relationship instead it found an object. If this is a polymorphic relationship please specify a type key. If this is an embedded relationship please include the DS.EmbeddedRecordsMixin* and specify the pictureExercises property in your serializer's attrs hash.

Looking at the ember data section in the ember inspector I can see all the top level exercise data is loaded correctly, the PictureExercises and VideoExercises are not.

How do I declare a type key on a polymorphic relationship in Ember.js?

I'm at a total loss and any help would be greatly appreciated. Thanks for your time.

//Create application store    
Gymbo.ApplicationAdapter = DS.FixtureAdapter;

//Set up models and relationships
    Gymbo.Exercise = DS.Model.extend({
        //id: DS.attr("number"), /*Ember will error if id is in model* /
        name: DS.attr("string"),
        description: DS.attr("string"),        
        pictureExercises: DS.hasMany("pictureExercise", {async: true }),
        videoExercises: DS.hasMany("videoExercise", { async: true }) 
    });


Gymbo.PictureExercise = DS.Model.extend({
        //id: DS.attr("number"),
        exerciseId: DS.attr("number"),       
        mimeType: DS.attr("string"),
        picFilePath: DS.attr("string"),
        picName: DS.attr("string"),
        exercise: DS.belongsTo("exercise")
    });

    Gymbo.VideoExercise = DS.Model.extend({
        //id: DS.attr("number"),
        exerciseId: DS.attr("number"),       
        videoName: DS.attr("string"),
        videoFilePath: DS.attr("string"),
        videoType: DS.attr("string"),
        exercise: DS.belongsTo("exercise")
    });

//UsersRoute
    Gymbo.ExercisesRoute = Ember.Route.extend({
        model: function () {
            return this.store.find("exercise");                      
        }
    });

//MOCK DATA
    Gymbo.Exercise.reopenClass({
        FIXTURES:
          [
            {
                "id": 101,
                "name": "Exercise 1",
                "description": "Description here",
                "pictureExercises": [
                    {
                        "id": 106,
                        "exerciseId": 101,                        
                        "mimeType": "PNG",
                        "picFilePath": "images\\strength",
                        "picName": "s001"                        
                    },
                    {
                        "id": 107,
                        "exerciseId": 101,                       
                        "mimeType": "JPG",
                        "picFilePath": "images\\strength",
                        "picName": s002
                    }
                ],
                "videoExercises": [
                    {
                        "id": 101,
                        "exerciseId": 101,                       
                        "videoName": "Test",
                        "videoFilePath": "Video\\strength",
                        "videoType": "mp4"
                    }
                ]
            },
            {
        "id": 102,
                //Data removed for brevity.....
            }
          ]
    });