validates_numericality_of

在针对float类型的字段quantity进行如下数量限制时
validates_numericality_of :quantity, :greater_than => 0

semantic_form_for中
= f.input :quantity, :step => 1

会发现这样的错误

A minimum value can not be determined when the validation uses :greater_than on a :decimal or :float column type.
Please alter the validation to use :greater_than_or_equal_to, or provide
a value for this attribute explicitly with the :min option on input().

修改为：
validates_numericality_of :quantity, :greater_than_or_equal_to => 0
或者
= f.input :quantity, :as => :string

validates_uniqueness_of
validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id]
限制组合字段的唯一性
:scope - One or more columns by which to limit the scope of the uniqueness constraint.