Improve nextId() generator

[pronebird]

Improves id generator to provide means to override default sequence.

I wasn't able to write tests, not sure how to mock the backend etc.

One of the tests would be to create model with arbitrary id and see if sessionData.nextId changes.

Also, in this change nextId() will always bump the counter, which is fine since it's not called anywhere else but within Model.

[pronebird]

Example usage with uuid:

import uuid from 'uuid';

class MyModel extends Model {

  static nextId() {
    return uuid.v4();
  }

  static advanceNextId(id) {
    // uuids are not deterministic
    // no-op
  }
}

[pronebird]

I am testing this and it seems to work fine in on of my apps.

[tommikaikkonen]

@pronebird I really appreciate the effort you put here. I ended up implementing this a little differently in #62 . For the pretty common UUID case, this should work:

class MyModel extends Model { }
MyModel.idAttribute = 'id';
MyModel.fields = {
    id: attr({ getDefault: uuid.v4 }), // might need uuid.v4.bind(uuid)
};

[pronebird]

@tommikaikkonen That's great! Thanks for your efforts too! Did you solve the problem with id sequencer for non-deterministic IDs such as UUID? Previous implementation tended to +1 UUIDs :)

[pronebird]

See comments above.