This is in reference to issue #1261, where Model->get_key() returns
the key from the $attributes instead of from the $original property.
This breaks the functionality of a model with a primary key that may
change, as the SQL generated will be something like:
UPDATE `model` SET `key` = 'new-key' WHERE `key` = 'new-key';
Which won't update the model in the database.
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