set controller attribute in promise before continue [ember cli]

Hi I have a doubt in how using promises in Ember. Basically I want to setup the attributes of my controller before running what is next the promise, my code:

export default Ember.Controller.extend({
  min: null,
  max: null,

  isValid: Ember.computed(function(){
    var self = this;

    var result = new Ember.RSVP.Promise(function(resolve, reject){
      var url = "..."
      Ember.$.getJSON(url).then(function(data){
        if (data[0] === 0){
          self.set('errorMessage', "It is a free book");
          return false;
        } else if (data[0] > 2000){
          self.set('errorMessage', "You are poor, go for next");
          return false;
        } else {
          console.log(data); # line 2 of console result
          self.set('min', data[1]);
          self.set('max', data[2]);
          return true;
        }
      });
    });
  }
  return result;
}),

actions: {
    save: function(){
        if (this.get('isValid')){
            console.log('save action is fired. -- VALID' + '--' + this.get('maxPrice')); # line 1 of console result
        } else {
            console.log('save action is fired. -- INVALID');
        }
    }
}           

});

This thing is when in the third if statement the code in save action is running before setting the attributes min and max inside the promise. Result in console:

> save action is fired. -- VALID--null
> [9, 1, 20]

Any idea how to set the values before the action go ahead?

Thanks,