finer-grained reactivity for nested objects

[dan335]

Template.hello.helpers({
user: function() {
return Meteor.users.findOne(Meteor.userId(), {fields: {"game.blah":1}})
}
})

If anything in Meteor.user().game changes then it invalidates this helper even though the fields specify that it only uses Meteor.user().game.blah. So if Meteor.user().var is updated then this helper will re-run.

https://github.com/dan335/fieldtest
http://fieldtest.meteor.com/
https://groups.google.com/forum/#!topic/meteor-talk/zg2cElH-Aqw

[glasser]

Yeah, this is a bug in the minimongo implementation of observeChanges with fields filtering. A reduced sinppet that demonstrates it:

var X = new Package.minimongo.LocalCollection;
var id = "asdf";
X.insert({_id: id, foo: {bar: 123}});

X.find(id, {fields: {'foo.bar': 1}}).observeChanges({
  changed: function (id, fields) {
    throw Error("but nothing changed!");
  }
});
X.update(id, {$set: {'foo.baz': 456}});

The problem is that wrapCallback inside observeChanges does the filtering AFTER diff.js does a top-level-only diff. While it tries to skip empty diffs, it's actually impossible for it to do so at that level: without the preimage, it can't actually know if the fields it is selecting have changed or not.

Probably the projection needs to be applied before the diff rather than after. Might be tricky though because some parts of the code rely on the fact that the elements of query.results are direct references to the docs in the collection object...

[glasser]

Turned out this was related to #3571. Fixed!

[stubailo]

This is awesome! I can't wait to start using this functionality to optimize my helpers.

[dan335]

Wow yes that is awesome! Can't wait for this next release.

[Slava]

Great work, should help to make Blaze predictable in performance.

[rclai]

Whoa! Does this mean nested fields have fine-grained reactivity now??

[glasser]

Well, it depends what you mean by that. observeChanges changed messages still have the same API: changes are reported at the top level. But if your observed query has a fields filter like {'a.b': 1}, then changed will now be correctly emitted if and only if a.b has changed.