Ember - async computed property or observer?

So I'm new to Ember and trying to work out the best way to set a computed property that depends upon related model data. I'll include the models below FYI.

I want to set the computed property to an asynchronous result (desiredOutcome). My question is how do I this? Or is it more appropriate to use an observer, as in this question.

score: function() {
    var self = this;
    var allscores = this.get('model.scores');

    //Find the scoreRecord that has the appropriate objective id (in this case just 1)
    var scoreRecord = allscores.find(function(item){
        var scoreID = item.get('id');

        return self.store.find('score', scoreID).then(function(scoreRecord){
            var objID = Number(scoreRecord.get('objective.id'));
            if (objID === 1){return true;}
        });
    });

    //Return the scoreRecord's score attribute
    var scoreID = scoreRecord.get('id');
    return this.store.find('score', scoreID).then(function(score){
        var desiredOutcome = score.get('score');
        console.log(desiredOutcome);
        return desiredOutcome;
    });
}.property('model.@each.scores', 'selectedObjective'),

Apologies for the poor choice of variable names etc...

My 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})