Accessing attributes of related models in the controller - ember.js

Question

I'm been asking a few questions around the same sort of lines and haven't managed to get an answer (perhaps I've been unclear), and just can't figure it out myself.
The quick version of the question is:
Can anybody shed some light on whether its possible to access a property of a related model from a controller?
Its a little tricky, so I'll try and explain the context.
I have the following models:
student
scores: DS.hasMany('score', {async: true}),
name: DS.attr('string')
objective
name: DS.attr('string'),
scores: DS.hasMany('score', {async : true})
score
scoreResult: DS.attr('number'),
objective: DS.belongsTo('objective', {async: true}),
student: DS.belongsTo('student', {async: true})
In my template, I can access attributes of related models with no problem. For example, something like this:
{{#each student in model}}
{{student.name}}
{{#each score in student.scores}}
{{score.objective.name}}
{{score.Result}}
{{/each}}
{{/each}}
What I ultimately want to do is create a "score" property on the student controller that loads the appropriate score/result when I choose an objective elsewhere. But I'm falling at the first hurdle.
Though I can access attributes of related models in the templates, I can't seem to in the controller. I'm expecting that I can do something like this in "student controller":
score: function(){
var selectedObjID = 5;
return this.get('model.scores').findBy('objective_id', 1).get('scoreResult');
I've tried every variation I can think of, including what I would have expected from reading the guides/api.
I feel like I'm missing something obvious - surely this should have been done enough before for there to be some documentation?
Its also important to do in the one route - I don't want details dealt with via another route.
--------------------------------Edit----------------------------
Thanks, I've tried adjusting your code a little, but I can't get it to work. Any chance you'd be able to help me correct it based on the model's above?
score: function() {
var scores = this.get('model.scores');
if (!scores) {
return "No scores";
}
var score = scores.findBy('objective', 1);
if (score === undefined) {
return "Not resolved/No score";
}
return score.get('scoreResult');
}.property('model.scores.#each.objective')

Answer 1

Though I can access attributes of related models in the templates, I can't seem to in the controller.
Based on this, I can take a good guess and say that your problem is unresolved promises. Ember Data doesn't always have synchronous access to related models, so it always returns a promise that will resolve later. More likely than not, you're trying to use that promise as if it's already resolved even though it's not yet. When writing computed properties based on Ember Data relationships, you should always write your properties as if you will be given empty data the first time through. Usually this means checking for null references and making sure the property updates when the promise resolves. To get to the point, here's your property that should work:
score: function() {
var scores = this.get('model.scores');
if (!scores) {
return;
}
var score = scores.findBy('objective_id', 1);
if (score === undefined) {
return;
}
return score.get('scoreResult');
}.property('model.scores.#each.objective_id')
The first time this property calculates this.get('model.scores') will be an empty PromiseArray, so you won't get any data from it. For those scenarios, just return and leave your property undefined (for now). But since the property is dependent on model.scores.#each.objective_id, the property will recalculate as soon as the PromiseArray resolves and the data is available. So this property won't have the right value until it runs 2 or 3 times, but it will eventually have the right value. The reason you don't get these issues when using the properties in a template is because Ember takes care of that for you.
Sorry for the long explanation, but there's a lot to learn in the area of unresolved promises and asynchronous computed properties (much more than I've posted). It's unfortunately one of those things that you just pick up as you learn Ember more. If I haven't made things clear or the above property doesn't work, let me know in the comments and I'll update and try to clarify.