Fixes #2653: Fixed the bug that unsetting an unpopulated AR relation would trigger exception (qiangxue)

fixes #1993

now supports mapped primary key when _id is part of source with an
alias.

Fixes #1579: throw exception when the given AR relation name does not match in a case sensitive manner.
Renamed `ActiveRecord::getPopulatedRelations()` to `getRelatedRecords()`

also fixed count behavior according to limit and offset.
fixes #1458

issue #1313

issue #1317