Add tests to test that through associations are not readonly, and we can update the records we retrive from the association

:strict

another value has been specified).

See the CHANGELOG for details.

Fixes #950.

This reverts commit c99d507f.

- instance_variables return symbols in 1.9
- there is instance_variable_defined? method

fixes #3183

when fetching ids for a collection, bypass preloading
to avoid the unnecessary performance overhead

Allows two models to use the same table but have different primary keys.

load has_many associations keyed off a custom primary key if that key is present but the record is unsaved