We were manually checking validation rules (like both relationable
objects are present, or they're both the same object) in the controller
and then using the `save!` method.
However, we usually use the `save` method (which checks all validations)
in a condition, and proceed depending on the result.
Now we're taking the same approach here. This means introducing a new
validation rule in the model to check whether both relationable objects
are the same, which is more robust than checking a condition in the
controller.
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