Here is your fiddle modified.
Im new to ember data. But from what I understand, you are merging 2 objects. I modified to convert the model object to array before merging.
var stream = Ember.A();
stream.pushObjects(txn.toArray());
stream.pushObjects(job.toArray());
return stream;
I also modified the property that your observing. Now Im observing the model data.
}.property('model.purchases.@each', 'model.tasks.@each'),