当一个关系因为已经加载而无法被修改时抛出。

class Task < ActiveRecord::Base
end

relation = Task.all
relation.loaded? # => true

# Methods which try to mutate a loaded relation fail.
relation.where!(title: 'TODO')  # => ActiveRecord::UnmodifiableRelation
relation.limit!(5)              # => ActiveRecord::UnmodifiableRelation