Populate sub-objects in fetch request

[anachlas]

No description provided.

[itsjoeconway]

at one point, was implemented but it was a bit confusing and the rules were a bit unclear. so we can take another look at this soon.

note that it'd be nice to have auto-generated async accessors for sub-objects instead of having to build the query ourselves

[itsjoeconway]

Just to start the convo, something like this:

var categoryTemplate = new Category();
var roomTemplate = new Room()..categories = [categoryTemplate];
var userTemplate = new User()..id = 1..rooms = [roomTemplate];

var users = await adapt.populateModelGraph(userTemplate);

Where users contains all users that have id = 1 (which we know to just be one for this particular value), and its properties will be populated, along with all of its rooms, and all of the rooms will have all of their categories.

Every property set on these objects would become filters (just like predicateObjects). An empty object in a relationship (or list of objects) indicates that you want them fetched.

Now, I really don't like this whole using model objects as templates for generating predicates, but its an OK temporary solution. Where it breaks down is when you need anything other than equality (<, >, like, etc.). What I'd really want is an automatically generated class for each model class. The auto generated class would have the same properties as the model class, except the properties would be a different type. Maybe some sort of PredicateMatcher. Its values could follow the Matcher syntax like in tests:

var userTemplate = new PredicateMatcher<User>();
userTemplate.name = beginsWith("B");
userTemplate.rooms.items = hasLength(10); // rooms is lazily allocated

var query = userTemplate.query;
var modelGraph = await query.fetch(adapter);