Fix bi-directional destroying of has_one :through

[airblade]

With a bi-directional has one through association, where the join model belongs to each parent with dependent: :destroy, destroying either parent should destroy the through record and the other parent.

class Left < ActiveRecord::Base
  has_one :middle, dependent: :destroy
  has_one :right, through: :middle
end

class Middle < ActiveRecord::Base
  belongs_to :left, dependent: :destroy
  belongs_to :right, dependent: :destroy
end

class Right < ActiveRecord::Base
  has_one :middle, dependent: :destroy
  has_one :left, through: :middle
end

However this only worked from one end. When destroying the "non-working" end, the join model was correctly destroyed but the other end was not.

In the example above:

• right.destroy correctly destroys its middle and its left;
• left.destroy destroys its middle but not its right.

The end which worked depended on the order of belongs_to statements in the join model.

This commit ensures that the no matter which end you destroy, the far end is destroyed (along with the join record).

Fixes #50948.

Checklist

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• [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
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• [x] Tests are added or updated if you fix a bug or add a feature.
• [ ] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.