allow order_for_rebuild to reference an associated table

[jakebrady5]

While this feature may be somewhat niche, I had a need for it. I thought I would PR the changes in case anyone else finds them useful.

The unscoped changes did not break any specs, but I do wonder about whether that might be a breaking change for some people. One upside from my perspective though, is that it gets closer to supporting other default scope gems like acts_as_paranoid.

Example Use Case

Using the spec models for consistency:

• Categories have many things through CategoriesThings.
• Things have many categories through CategoriesThings.
• Existing behavior was that when viewing a list of Things for a given Category, that list is return in alphabetical order by a field (body).
• You want to add awesome_nested_set to CategoriesThings to allow custom ordering. You also want to preserve the alphabetical order for existing records until a user explicitly changes it.
• Changes were needed in order to reference a sort order belonging to a different database table.

[stale[bot]]

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