this problem occurs when joining active record but not selecting the
primary key field. records get populated but they do not have primary
key value populated which results in only one record left over in the
result.

issue #2852

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

- refactored elasticsearch AR to make type and index available in
  instantiate(), issue #1313

fixes #2281

Fixes #2016: removed ability to declare scopes in ActiveRecord leaving it to ActiveQuery. Changed documentation accordingly.

fixes #1993

Renamed `attributeName` and `className` to `targetAttribute` and `targetClass` for `UniqueValidator` and `ExistValidator`.
Refactored UniqueValidator and ExistValidator.