TS::Cache#compute_if_absent guarantees that only a single thread will get to execute the provided block for a given key.

Also renaming ivar @odd_or_even to @odd

In order to build associated records for owners which has not been saved
need to get where values to use as default attributes.
But for new record owner uses `ActiveRecord::NullRelation` which
override `where_values_hash` to return empty hash stub.

`where_values_hash` is not used to invoke any sql query, but good to
build others chains (even will be never executed) like:

```ruby
  post          = Post.new
  admin_comment = post.admin_comments.build

  assert_equal 'Admin', admin_comment.author
```

Closes #11376, #11676, #11675

.find([1]) should return an Array of entries, even when a invese object is in memory already

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