Lazy loading relationships about orbit

[dgeb]

@opsb You are correct that relationships are synchronously populated from the source. However, it's possible to asynchronously fetch them, as described in the README:

A model's relationships can be re-queried by calling reload() on a relationship object. This triggers the associated findLink call and returns its promise. Related records will be updated through transformations to the underlying source. Any changes should be reflected by the time the promise resolves.

For example:

planet.get('moons').reload().then(function() {
  console.log(planet.get('moons.length'));
});

Please re-open if you have any problems.

[opsb]

I did see that, if a view is showing a nested object graph though it's a bit cumbersome to have to go through the graph and call reload() on each relationship. In my case I'm now solving it by adding firebase listeners to each relationship when a record is loaded. That way they're loaded immediately and keep up to date with server side changes. This works fine for me but I think lazily loaded relationships would be worth addressing at some point, in particular with the JSON-API adapter.

[dgeb]

Eager fetching is something that needs to be carefully controlled or else it will get out of control. However, I can imagine it can be controlled given the right configuration settings.

One scenario I could imagine is providing hooks to specify different include params in the JSONAPISource, depending on the type requested. Furthermore, we could extend schemas in orbit to allow per-source configuration data to be stored for each type. And that configuration data could flow from model definitions in ember-orbit.

[opsb]

Firebase makes great claims about being very efficient with regards to adding a large number of listeners, I'm building the Source based on this assumption initially but I'll see how it runs with my app and make it more intelligent if necessary.

With regards to specifying include params on a per type basis, I'd say that one of the major issues I had with ember-data is that it's not possible to specify includes on a per-scenario basis. I found this an extremely difficult juggling act as it meant loading up lots of unnecessary information when showing list views for instance. Bearing that in mind and considering that relationships are implemented as proxies it might be nice to allow something like:

persons.projects.includes(['members']);

I've had rails like scopes for relationships in the back of my mind for a while now, it's something that I'll take a look at when I've got some time.