RelationContract (hasMany) and FieldRelationContract (belongsTo) accept
ClearOnChange, a list of nullable columns set to NULL whenever the relation
changes the foreign key: a hasMany link and unlink (deferred ones included)
clear them on the child, and a form save that moves a belongsTo key to
another record or to none clears them on the record unless the same save
writes the column through its own field. Boot refuses the option on a
belongsToMany and for a missing, non-nullable, foreign-key or primary-key
column. The zero value changes nothing.
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