owners once

the @associated_records_by_owner ivar

code

conditionals from the individual preloaded classes

If you are passed an empty hash, then assign_attributes doesn't need to
do any work and can just return early. This should fix the failing Agile
Web Development tests.

ActiveRecord::ConnectionAdapters::Column.string_to_time method respects string with timezone. Closes #12278.

and possibly different rules for finding those objects

Make AM::Dirty less dirty to plugin into AR or other library

This will make easier to hook protected_attributes gem in our code
without making that gem fragile to change in Rails code base.

Closes #12243

objects