ActiveRecord: scoped `validates_uniqueness_of` allows one null value per scope
As you most likely know validates_uniqness_of :foreign_id does not allow nil values by default.
To allow nil one has to set the :allow_nil => true option.
Very unexpected scoping this validation will not raise an error if foreign_id set to nil for the first created record of this kind.
validates_uniqueness_of :foreign_id, :scope => :another_column # allows foreign_id to be nil
Without a validation for presence of foreign_id now unusual records could be created.
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